Unable to add, delete or manipulate Websites

today I wanted to add just another Website to my PIWIK Dashboard. Unfortunately PIWIK displayed this error:

An unexpected website was found in the request: website id was set to ‘’ .

Furthermore it said:

SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry ‘0’ for key ‘PRIMARY’

Now I can’t access “All Websites” and I even can’t add or delete Websites I added in the past. Has anybody experienced something similar? I welcome your support or response. With PIWIK Diagnosis a critical problem is displayed: „always_populate_raw_post_data=-1“ — I even don’t know exactly how to fix that.

I welcome your response.