systemd-timesyncd stopped working

[welp]

Help! I'm really out of ideas... Since December 18th, systemd-timesyncd no longer works, for whatever reason there are time-outs to the ntp servers:

here is what I have in the log (date/time is set manually):

$this->bbcode_second_pass_code('', 'Jan 01 19:01:13 pi systemd-timesyncd[4758]: Timed out waiting for reply from 87.97.157.120:123 (3.arch.pool.ntp.org).
Jan 01 19:01:13 pi systemd-timesyncd[4758]: Using NTP server 84.54.128.8:123 (3.arch.pool.ntp.org).
Jan 01 19:01:23 pi systemd-timesyncd[4758]: Timed out waiting for reply from 84.54.128.8:123 (3.arch.pool.ntp.org).
Jan 01 19:01:23 pi systemd-timesyncd[4758]: Using NTP server 84.43.191.4:123 (3.arch.pool.ntp.org).
Jan 01 19:01:33 pi systemd-timesyncd[4758]: Timed out waiting for reply from 84.43.191.4:123 (3.arch.pool.ntp.org).
Jan 01 19:01:34 pi systemd-timesyncd[4758]: Using NTP server 216.239.32.15:123 (time1.google.com).
Jan 01 19:01:44 pi systemd-timesyncd[4758]: Timed out waiting for reply from 216.239.32.15:123 (time1.google.com).
Jan 01 19:01:45 pi systemd-timesyncd[4758]: Using NTP server 216.239.34.15:123 (time2.google.com).
Jan 01 19:01:55 pi systemd-timesyncd[4758]: Timed out waiting for reply from 216.239.34.15:123 (time2.google.com).
Jan 01 19:01:56 pi systemd-timesyncd[4758]: Using NTP server 216.239.36.15:123 (time3.google.com).
Jan 01 19:02:06 pi systemd-timesyncd[4758]: Timed out waiting for reply from 216.239.36.15:123 (time3.google.com).
Jan 01 19:02:07 pi systemd-timesyncd[4758]: Using NTP server 216.239.38.15:123 (time4.google.com).
Jan 01 19:02:17 pi systemd-timesyncd[4758]: Timed out waiting for reply from 216.239.38.15:123 (time4.google.com).
Jan 01 19:02:50 pi systemd-timesyncd[4758]: Using NTP server 84.43.191.2:123 (0.arch.pool.ntp.org).
Jan 01 19:03:00 pi systemd-timesyncd[4758]: Timed out waiting for reply from 84.43.191.2:123 (0.arch.pool.ntp.org).
Jan 01 19:03:00 pi systemd-timesyncd[4758]: Using NTP server 212.70.148.16:123 (0.arch.pool.ntp.org).
Jan 01 19:03:10 pi systemd-timesyncd[4758]: Timed out waiting for reply from 212.70.148.16:123 (0.arch.pool.ntp.org).
Jan 01 19:03:10 pi systemd-timesyncd[4758]: Using NTP server 212.70.148.13:123 (0.arch.pool.ntp.org).
Jan 01 19:03:21 pi systemd-timesyncd[4758]: Timed out waiting for reply from 212.70.148.13:123 (0.arch.pool.ntp.org).
Jan 01 19:03:21 pi systemd-timesyncd[4758]: Using NTP server 212.233.218.5:123 (0.arch.pool.ntp.org).
Jan 01 19:03:31 pi systemd-timesyncd[4758]: Timed out waiting for reply from 212.233.218.5:123 (0.arch.pool.ntp.org).
Jan 01 19:03:32 pi systemd-timesyncd[4758]: Using NTP server 46.40.123.212:123 (1.arch.pool.ntp.org).
Jan 01 19:03:42 pi systemd-timesyncd[4758]: Timed out waiting for reply from 46.40.123.212:123 (1.arch.pool.ntp.org).
Jan 01 19:03:42 pi systemd-timesyncd[4758]: Using NTP server 87.120.164.97:123 (1.arch.pool.ntp.org).
Jan 01 19:03:52 pi systemd-timesyncd[4758]: Timed out waiting for reply from 87.120.164.97:123 (1.arch.pool.ntp.org).
Jan 01 19:03:52 pi systemd-timesyncd[4758]: Using NTP server 84.238.140.68:123 (1.arch.pool.ntp.org).
')

I also tried reverting to ntpd and netctl as opposed to systemd-timesyncd/networkd, however that doesn't work either, it simply doesn't update the time, I don't see any indication in the logs why, at least timesyncd shows time-outs. There is no problem pinging the servers:

$this->bbcode_second_pass_code('', 'ping 1.arch.pool.ntp.org
PING 1.arch.pool.ntp.org (212.70.148.20) 56(84) bytes of data.
64 bytes from 212.70.148.20: icmp_seq=1 ttl=249 time=696 ms
64 bytes from 212.70.148.20: icmp_seq=2 ttl=249 time=78.4 ms
64 bytes from 212.70.148.20: icmp_seq=3 ttl=249 time=75.2 ms
64 bytes from 212.70.148.20: icmp_seq=4 ttl=249 time=73.1 ms
^C
--- 1.arch.pool.ntp.org ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 3002ms
rtt min/avg/max/mdev = 73.119/230.898/696.757/268.970 ms
')

Restarting systemd-networkd (as suggested here) doesn't help at all.

[welp]

The following shows up in the log when I restart timesyncd:

$this->bbcode_second_pass_code('', 'Jan 01 19:28:22 pi systemd[1]: Reloading.
Jan 01 19:28:34 pi systemd[1]: Stopping Network Time Synchronization...
Jan 01 19:28:34 pi systemd[1]: Starting Network Time Synchronization...
Jan 01 19:28:34 pi systemd[1]: Failed to reset devices.list on /system.slice: Invalid argument
Jan 01 19:28:34 pi systemd[1]: Failed to reset devices.list on /user.slice: Invalid argument
Jan 01 19:28:34 pi systemd[1]: Started Network Time Synchronization.
Jan 01 19:28:34 pi systemd-timesyncd[3577]: Using NTP server 91.134.37.17:123 (0.arch.pool.ntp.org).')

Could this point to a problem? What?!

[krabat]

If something stops working for apparently no reason it's always a good idea to look for changes that may have taken place just before the problem started manifesting.
This comprises both configuration changes and updates.

You badly have to post more information regarding your system. Hardware, kernel in use etc.

Incompatibilities of latest systemd with rather old kernels are a common problem. Those "invalid argument" messages suggest it could be something similar here.

Other network protocols - HTTP, SSH, ... - are working without issues?

[welp]

The hw is: raspberry pi B; it connects to the internet through a Huawei E220 modem (umtskeeper + sakis3g); the only other hw attached to it is a USB receiver for a weather station. Same hw, no changes, since I set it up some months ago.

$this->bbcode_second_pass_code('', 'uname -a
Linux pi 3.12.35-1-ARCH #1 PREEMPT Tue Dec 23 07:14:51 MST 2014 armv6l GNU/Linux')
How weird... it's showing Dec 23, whereas:

$this->bbcode_second_pass_code('', '# timedatectl status
Local time: Tue 2015-01-06 13:40:23 EET
Universal time: Tue 2015-01-06 11:40:23 UTC
RTC time: n/a
Time zone: Europe/Athens (EET, +0200)
NTP enabled: no
NTP synchronized: no
RTC in local TZ: no
DST active: no
Last DST change: DST ended at
Sun 2014-10-26 03:59:59 EEST
Sun 2014-10-26 03:00:00 EET
Next DST change: DST begins (the clock jumps one hour forward) at
Sun 2015-03-29 02:59:59 EET
Sun 2015-03-29 04:00:00 EEST')
(I had to set «timedatectl set-ntp false» in order to set the date manually, neither ntpd nor timesyncd work when I enable them as of today 06.01.2015)

Indeed SSH works, I have a service that creates a reverse SSH connection to my home machine so I can in turn connect to the R-Pi. (I can't access it directly as it's behind the carrier's NAT)

The only thing that runs on the system is weewx — a python weather software, it uploads weather data to Wunderground. FTP and RSYNC also work. I don't have a firewall.

Am I missing anything else?

[welp]

As I was out of ideas (and didn't know how to troubleshoot access to NTP servers, pinging worked but then NTP is over UDP etc) I decided to re-install everything hoping it would work.

Well, it did not.

Then I simply swapped the SIM card with a different carrier and voila — suddenly NTP works! I even tested with a third carrier to make sure it really was a carrier-specific issue! So, it appears for whatever strange reason one of the three mobile operators here had decided to block NTP traffic, my attempts of getting their cust. serv. get back to me on the issue were completely futile.