Strange error with no traceable cause.

[crimsonsunrise]

I'm working on a large score using 2.21.0, parts all separated into different files and such, suddenly, after adding a bar for the 3rd clarinet, this shows up:

Parsing...

Interpreting music...[8][16]ERROR: In procedure ly:trampoline:

ERROR: Wrong type argument in position 3 (expecting Translator): #<freed cell 0x7fe355b5e220; GC missed a reference>

I have no idea why this is happening. All the other parts add fine but, as soon as the bar is added for that particular instrument, the error shows up.

However, if you engrave with verbose output on Frescobaldi enabled...the error doesn't show up. Disable that and do normal engraving...Same error.

Any ideas on what's causing it or it might be a Fescobaldi bug?