DB Error: constraint violation on anonymous user contribution/membership registration after 5.8.2 -> 5.9.0 update
This is a core bug and Eileen has send a PR for the fix. Will you be able to try by applying the patch from here? 

If the patch works for you than please comment your test result on PR so that it will speed up process of merging into core and having it available in next release.

Cheers

Pradeep