First we need to establish if Synth1 Librarian is the problem, or if there's a bigger issue. Try this:
1) Load up Synth1 VST in your DAW (not via the Librarian) 2) Click opt, and change the ExtBank folder to a temporary location (e.g. C:\) 3) Click OK 4) Close down your DAW completely, so that Synth1 VST is unloaded from memory 5) Now restart your DAW, and reload Synth1 VST 6) Click opt, and check the the ExtBank folder.
If the value isn't C:\, you've got some kind of Windows issue. Does your Windows user name contain accented characters? If so, that could cause problems.