Hi Ahmed,
Unfortunately the error is a bit vague. What I'd suggest is to open a ticket with Campaign Support and provide the following into:
1) Rest API request/response including source and endpoint
2) When this first started?
3) If this ever worked. If so, provide previously working examples.?
4) What events did you delete and what steps did you take exactly?
It's possible that if this started around the unpublish process that there might be a hung or long running queries which are locking database tables. Support should be able to review and work with Adobe Ops to review from the backend DB.
Regards,
Craig