@esprit-libre Thunderbird probably doesn’t use the same REPORT calendar-query request, so that the server error (which occurs when the server tries to process the events in order to process the REPORT calendar-query request) doesn’t occur.
You can create a new calendar, upload a single event, and then have a look whether the problem occurs. If it does, you have a look at the uploaded event, and the current event in the server’s database.
However, it’s clearly a server problem, so I suggest to report it to the server vendor and continue discussion there. Which server do you use?
BTW: Android doesn’t support events without end dates. All-day events will become assigned end := start + 1 day, the other events end := start.