Bug 434211

SUMMARY
I self-host my calendar using Etebase, the Etesync server. There are three clients connecting to my server: My Android phone using the Etesync Android app, Korganizer and Kaddressbook on my laptop using the native Etesync integration, and the Etesync web app which I'm using for testing purposes.

After I initially set up my phone and my laptop, things worked perfectly for a few weeks, but then I started getting no synchronization. I went on the web app and was able to confirm that there was synchronization between my phone and the Etesync web app both ways, so I know my server works. However, changes made anywhere else do not appear on Korganizer and Kaddressbook, and changes made on Korganizer and Kaddressbook do not appear on the web app or my phone, so it is most likely Korganizer and Kaddressbook that are misbehaving.

Additionally, I noticed the following stuff in my journalctl -xe:
    Mar 09 14:37:09 arch-btw akonadiserver[857]: org.kde.pim.akonadiserver: Cannot connect to agent instance with identifier 'akonadi_etesync_resource_2', error message: ''
    Mar 09 14:37:09 arch-btw akonadiserver[857]: org.kde.pim.akonadiserver: Cannot connect to agent instance with identifier 'akonadi_etesync_resource_2', error message: ''
    Mar 09 14:37:09 arch-btw akonadiserver[857]: org.kde.pim.akonadiserver: Cannot connect to agent instance with identifier 'akonadi_etesync_resource_2', error message: ''
    Mar 09 14:37:09 arch-btw akonadiserver[857]: org.kde.pim.akonadiserver: Cannot connect to agent instance with identifier 'akonadi_etesync_resource_2', error message: ''
    Mar 09 14:37:09 arch-btw akonadiserver[857]: org.kde.pim.akonadiserver: Cannot connect to agent instance with identifier 'akonadi_etesync_resource_2', error message: ''
    Mar 09 14:37:09 arch-btw akonadiserver[857]: org.kde.pim.akonadiserver: Cannot connect to agent instance with identifier 'akonadi_etesync_resource_2', error message: ''
    Mar 09 14:37:09 arch-btw akonadiserver[857]: org.kde.pim.akonadiserver: Cannot connect to agent instance with identifier 'akonadi_etesync_resource_2', error message: ''
    Mar 09 14:37:09 arch-btw akonadiserver[857]: org.kde.pim.akonadiserver: Cannot connect to agent instance with identifier 'akonadi_etesync_resource_2', error message: ''
    Mar 09 14:37:09 arch-btw akonadiserver[857]: org.kde.pim.akonadiserver: Cannot connect to agent instance with identifier 'akonadi_etesync_resource_2', error message: ''
    Mar 09 14:37:09 arch-btw akonadiserver[857]: org.kde.pim.akonadiserver: Cannot connect to agent instance with identifier 'akonadi_etesync_resource_2', error message: ''
The number 857 changes on every reboot, but it's usually in the mid 800s, and I'm pretty sure it's the PID of the akonadi server. These messages appear to always come in batches of 10, and those batches appear to consistently show up either almost exactly 5 minutes or 10 minutes (though whether it's 5 or 10 minutes seems random) after the previous batch.

I initially asked this to Etesync's Matrix room, and was told to file a bug report here.

STEPS TO REPRODUCE
1. Set up Korganizer and Kaddressbook to sync with native Etesync integration 
2. Use it for several weeks

OBSERVED RESULT
No synchronization between Korganizer/Kaddressbook and Etesync server

EXPECTED RESULT
Synchronization between Korganizer/Kaddressbook and Etesync server

SOFTWARE/OS VERSIONS
Windows: N/A (don't use Windows)
macOS: N/A (don't use macOS)
Linux/KDE Plasma: Linux 5.4.96-1-lts provided by Arch's linux-lts package, KDE Plasma not installed
(available in About System)
KDE Plasma Version: N/A
KDE Frameworks Version: N/A (I think)
Qt Version: I don't know which specific package you want the version of, so here's all of them.
    qt5-base 5.15.2-5
    qt5-declarative 5.15.2-1
    qt5-graphicaleffects 5.15.2-1
    qt5-location 5.15.2-2
    qt5-multimedia 5.15.2-1
    qt5-networkauth 5.15.2-1
    qt5-quickcontrols 5.15.2-1
    qt5-quickcontrols2 5.15.2-1
    qt5-script 5.15.2-1
    qt5-speech 5.15.2-1
    qt5-svg 5.15.2-1
    qt5-tools 5.15.2-3
    qt5-wayland 5.15.2-1
    qt5-webchannel 5.15.2-1
    qt5-webengine 5.15.3-2
    qt5-x11extras 5.15.2-1
    qt5-xmlpatterns 5.15.2-1
Packages that the template doesn't ask for but I think may be relevant:
    kdepim-runtime 20.12.3-1 (Note: This was not an option in the "Version" field on the bug report form, so I left that field at "unspecified".)
    libkdepim 20.12.3-1
    korganizer 20.12.3-1
    kaddressbook 20.12.3-1
    akonadi 20.12.3-1
    akonadi-calendar 20.12.3-1
    akonadi-contacts 20.12.3-1
    akonadi-mime 20.12.3-1
    akonadi-notes 20.12.3-1
    akonadi-search 20.12.3-1
    libakonadi 20.12.3-1


ADDITIONAL INFORMATION
None that I can think of