Unable to view CiviCRM contacts anymore
It looks like from your backtrace, its a bug in GDPR extension because its not possible to replicate in CiviCRM. Your system has many custom group for activity, and the extension code breaks when trying to retrieve results, Mysql supports max 61 joins in a query. 

Possible solution:


Disable custom group for activity which is not required.
Restructure your custom group by moving custom fields into new/existing custom group and delete the ones not required. This is bit risky and will take lots of effort since UI may not allow you to move custom field if its defined for subtype. And if you have already used this field in webforms than probably you may need to retain the custom field ID.
Apply the fix from here. 


Thanks

Pradeep