[WinError 123] The filename, directory name, or volume label syntax is incorrect

[mcsdodo]

Mod manager installed in: D:\Games\TWR3_Mods
witcher.exe D:\Games\TheWitcher3\bin\x64

[Systemcluster]

Thank you for reporting this.

To make sure, this error appears while trying to install a mod? Would you be able to run the source version with python directly and see if a more detailed error description appears in the console?