welcome to the notepad++ community, @Ak-M
Thoughts ?
not really, at least not yet.
as both version 7.6.4 and 7.6.6 are not code signed, and 7.6.4 works, i would exclude a security policy, that prevents unsigned code to access your data files.
if you have time and dare testing pre-release versions, you could also try the current 7.7 release candidates from >>> here <<<, to see if they would work on your system. they are code signed again, so maybe they could work, if it is a security setting or protection software that causes this.
(portable .zip versions are also available, for testing without the need of installing them)
best regards.