One of the customers had experienced the error upon entering Approval Requests page:

And it happened after metadata changes that looked reasonably safe.

So what happened?

It turns out that if you have an entity that is a part of workflow (draft, approve, …, publish) and you already have published some of them -- deleting attribute which is an embedded array of other entities would lead to this kind of error.

Luckily, there is a way to fix it if you have access to MMM database:

  delete from "mdArbitraryReference_r" where id in ( select id_i from "mdArbitraryReference_q" where "referencedNodePath_s" not in (select id from "_MmdDictionary" where type = 'PATH'));

  delete from "mdArbitraryReference_q" where "referencedNodePath_s" not in (select id from "_MmdDictionary" where type = 'PATH');

These SQL queries fix DB inconsistency removing invalid records from arbitrary references tables.

I would still suggest to consult with support if you encounter such an error first, though. :)