This could happen if there are render files inside the Library. The solution requires some deep digging into the Library files, so be sure to make a backup of the Library before proceeding! Steps to do in case Postlab is not allowing you to import a Library because it may contain render files inside:
Duplicate the Library on Finder.
Right click on the library and choose "Show Package contents"
Disclose all folders inside the Library and find all "Render Files" folder and delete them.
Try to import on Postlab.
Create a brand new Library on Postlab and Start Editing.
Open Library that's not being imported and drag and drop all events to the Postlab created Library.