Openproject 11.0.0(PostgreSQL) on Ubuntu 16.04.7 LTS Kernel 4.4.0-186-generic

Whenever I try to create a new Task, Milestone etc I will receive the following error:

"PG::UndefinedFunction: ERROR: function array_append(integer[], bigint) does not exist LINE 16: THEN array_append(path, relations.from_id) ^ HINT: No function matches the given name and argument types. You might need to add explicit type casts."

Any hints on what I can try to resolve the issue?

Thank you

Marko