Could not open file for write: Input/output error - when making a project in the Cryptomator mount folder

I’m using Cryptomator Linus desktop with the Mega cloud service to store an encrypted PROJECTS folder in the Mega cloud on a Linux machine. When I decrypt the vault it appears to be mounted at the following mount point : /home/user/.local/share/Cryptomator/mnt/PROJECTS/
I am able to use this decrypted folder no problems and all the files appear to be accessible, however the compliler (and other associated apps invoked during the make process) returns an error related to a particular file, namely :
@E:Could not open file ‘xpartition.sxml’ for write: Input/output error.
The file itself is not important here, it is rather the fact that some part of the make process fails as it cannot open a particular file ‘xpartition.sxml’ and make then halts with the above error.
To test if this is an issue otherwise, I just copied the PROJECTS folder to another location in my /home/user/Mega folder (same one that is synced by Mega) and then tried to make the project from that location. This resulted in a successful compilation etc. - ie make completed to the end as expected, no errors !
So my question is what is causing the make I/O error that appears to be specific to the compilation occuring on the files in the decrypted mounted folder, whereas it does not seem to be a problem if the same, identical project files are compiled in another folder in the /home/user/ directory ??
I have checked the chmod access and in both case (Cryptomator mount point, and another folder in /home/user/) they are the same, and owner (me) has full rwx access, so this would appear to rule out this potential issue.
Could anybody offer a suggestion as to why this @E:Could not open file ‘xpartition.sxml’ for write: Input/output error occurs when making the project in the Crypto mounted folder ?
Preferably I would not have to copy the PROJECTS folder before being able to correctly make it.
thanks in advance, MM