@Hendrik Peter I'm really happy that this might be fixing your problem. I don't believe it is the same post as sbot crashes before gossiping and until you fix it, it won't start.

Did this happened just after you posted a message? Was it from which client? I've patched it in the current patchfox version (the one that is available from the store) and the update is automatic. If this happened after using some other client then we need to investigate. It is caused by a broken mentions array.