There were Linux-based tests with very long (cca 3.5kbyte) paths in SquareLine Studio save/load/import/export, all worked fine, so the path seems not be limited to a certain length in SquareLine Studio itself.
But on Windows default max. path length is 260 characters, to circumvent this the longer paths should start with \\?\ mark (only supported since Windows 10 rev1608). AFAIK support for this will be added in SquareLine Studio soon (probably next release in April).
Until that if there are too long filenames or problem with OneDrive ‘subfolders’ as was suggested by one, you should export to a folder that’s not residing on OneDrive.