Outdated and vulnerable Synapse version
Tedomum.net is running Synapse 1.88.0, while the current stable release is 1.95.1 at the time of writing.
There have been multiple reported security advisories since that release including the aforementioned 1.95.1.
Tedomum.net also cannot be used to join rooms using room version 11 since that requires at least Synapse 1.93.0.
I hope you will upgrade soon. Thank you for running the homserver.