It also depends where
your document is placed!
If it is SD card, please, check if it is not write protected. If you use micro SD card in SD adapter it is quite easy to switch write protect on while inserting the card in M92. It has happened to me several times.
The scribles (and annotations??) are stored in hidden .onyx folder, which is placed at location where the document is. Therefore if the storage is write protected nothing can be stored.
Wouldn't it be possible to make a check and warn the user? (When the file is opened and also when the card is inserted.)
Well, the best would be to do the proper error handling and if writing is to be done, but not possible, inform the user.
And good idea might be if saving to proper place is not possible to save it to other location and allow the restoration when writing is possible.
If several hours of work is lost, no-one is happy.
Some people use M92 for work, not just leisure.