Hope someone knows a quick fix, this is a weird one.

Win10 (obviously).
Registry-File on a network share.
If I execute it without mapped drive (ie directly through the share) - OK.
If I map the share to a letter, and try to execute:
Registry Editor
Cannot import O:\test.reg: Error opening the file. There may be a disk or file system error.

I am a user and a member of the local Administrators group.
UAC is turned off.

..and: I can execute executable files normally. Some need Admin rights, but not all.

Had previously Win7->Win10 upgrade, now decided to install clean.

Many thanks afront.