Answer
One worthwhile investigative lead: check Application Base Setup > System Setup > Object Connection Transformations for anything added or changed recently, particularly entries where 'System Defined' is unchecked (indicating a custom, non-standard transformation) — a newly added transformation causing a circular reference in how object connections resolve is a plausible cause for a cursor-already-open error appearing suddenly in this specific call chain. This is a diagnostic direction to investigate rather than a guaranteed fix, so it's still worth pursuing in parallel with a support case if the error is actively impacting users.