How to reproduce: Create/rename an .sr file to e.g. "ü.sr", double-click it, PV will show an error and not load the file.
Pretty sure this needs to be fixed in NSIS (not PV itself), since manually opening the file from within PV works fine. Linux has no issues with "ü.sr" either.
It's probably just a simple matter of properly quoting or escaping filenames in the NSIS file extension association setup.