Thank you for the Trouble Zip, Neurokraft.
It indicates that you've had a few crashes with BookMacster, one of them today and one last week. I fixed the bug which was causing that crash, for the next version of BookMacster, but I don't think this is related to the issue you are seeing.
I had a couple users report a similar issue during the past year. Both users were
power users who had a lot of system extensions and hacks installed. I even opened a support incident with Apple, but the Apple engineer could not figure it out either. Then, both users reported that the issue disappeared after they upgraded to macOS 10.12 Sierra.
I see that have been using 10.12, and don't see evidence of any system extensions or hacks in your Trouble Zip, except for
SwitchResX, which is injecting code into BookMacster. So I found and installed SwitchResX, and relaunched BookMacster a few times after changing some SwitchResX settings, but did not have any trouble opening document windows.
At your convenience, please
log into a clean Macintosh user account, and see if BookMacster works as expected with
File >
New Bookmarkshelf. Also, think about if you have installed any apps which might be considered system extensions or hacks since the last time that BookMacster worked properly.
Let us know what you find, and we'll continue.