Saving event crashes when registration start is placed before current date

One-sentence description

Saving event crashes when registration start is placed before current date

Current behaviour / Reproducing the bug

1. Create an event
2. Set registration start date in the future
3. Save event
4. Change registration date before current date
5. Crash (locally I get a Fatal Python error: Cannot recover from stack overflow., which looks like infinite recursion

Expected behaviour

Setting registration start date before current date opens registrations