Students get errror writing to database

[hgeorgsch]

The bug #165 seems to persist.
I incidentally observed a student getting it, and he said he had seen it about ten times. There is no obvious pattern to when this occurs.
The only plausible causes I can think of are, (1) race conditions between students, and (2) load.

[hgeorgsch]

Solving #150 first may help with this.

[hgeorgsch]

Let's hope that this has been resolved by #192
In any case it cannot be systematically reproduced and we will have to wait for new reports before it can be researched further.