It seems that when having big amount of virtual URI mappings (500+) in light modules, loading and processing of mappings get randomly interrupted with misleading error
The error is posted in the logs despite the fact that same yaml file loads on different instance w/o any issues and class used for the mapping is default mapping included in Magnolia installation OOTB.
Error also appears at random when reloading of definitions is forced by adding new one.
And at random might get fixed (or not) by restart of the instance.
There are no errors reported on any other of the existing mappings thus making it impossible to figure out if any other file might be the cause. There is also no known debug setting that would help ti triage the issue nor means to verify all existing mappings to figure out if there is conflict elsewhere.
Details of existing investigation in linked tickets: