Error > httpd 500 error response: {"error":"unknown_error","reason":"undef"} after upgrade from 2.0.0 to 2.1.1

[petekun]

Getting the below error on the couch log after upgrading from couchdb version 2.0.0 to 2.1.1

[error] 2018-04-25T09:54:26.929356Z [email protected] <0.5554.0> -------- httpd 500 error response:
{"error":"unknown_error","reason":"undef"}