I think that's the problem, for some reason accessing the NAS from the Mac is slow. So for bigger libraries it takes enough time to make the app timing out.

But why? To be honest I don't know. You can try copying a file (or a folder) from the NAS to the Windows 10 machine and see how long does it take, and then do the same in your Mac. Try something substancial (5 GB of data or so).

I had this same kind of problem using a library on an external drive. Maybe it was ejected wrong, I can't say what happened. Things have been working fine until now.

Here's what happens:

When I try and open a file through the YACLibrary, I get the error popup Format not supported and the file asking to Press 'O' to open comic. This actually opens the file and we're good to go. When I try and open this file from the YAC Library again it works.

I can open files normally from the YAC Library my local drive. But I can't load up files to my iPad from the external drive. (I had issues locally but that appeared to be illegal characters in the filename.)

So I unmounted and mounted the drive; ejected the drive, turned it off, and unconnected it before remounting; I tried to figure out how to add the 'nolock' option when mounting (couldn't quite figure that one out for Mac OS X); I updated to the latest dev version (the xml scanner works great, BTW).

To troubleshoot, I copied a local directory that had files that were working to the external drive. The copied directory failed the same way. When I compared the two directories, the local permission was for "staff" and the external directory had permissions for "wheel" (superuser group). Trying, there was a @ symbol at the end of some of the permissions. These turned out to be the files I initially failed to open, but was able to open fine after "manually" opening them after the error.returned an empty
If I figure it out, I'll post. Otherwise has this happened to anyone else?

OK. The answer was actually pretty simple. You just need to give YACLibrary Full Disk Access.

Click the Apple icon → Click "System Preferences" → Click "Security & Privacy" → Click "Privacy" → Click on Full Disk Access. Unlock and select YACLibrary and YACReader.

There's probably a better more secure way to do this, but tonight I'm going to have to just trust Luis. Changing the Files and Folder Privacy setting wasn't enough.