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.
Could you please take a look at high PPI friendliness? It seems like it reports wrong windows size to a host. I'm using 4K monitor, with ppi scaling set to 175%. When the plugin I run the plugin in Reaper in a native mode (without bridging), everything looks properly (fonts are scaled and sharp) but rendered window is horizontally too small. see screenshot
I've also tried to run the plugin in bridged mode. This time the entire window is displayed, but fonts suffer from the issue of non-high-ppi-ready-application, which renders by blurred fonts (those are rendered to bitmaps and upscaled by operating system. See screenshot
This issue might be however related to Reaper bridging implementation.