The system cannot find the path specified if path on Windows contains lots of characters
It seems that when comparing directories the Windows path limit is in place regardless of the registry override.
I have some java projects whose packages tend to be lengthy (more than 250 characters) and Meld seems to not find the files. The length limit of Windows 10 has been overridden and the problem is still present.
A quick workaround would be to use SUBST to shorten the path but it's tedious to do this for many projects.
I'm using Meld version 3.20.2 on a Windows 10 20H2.