TransitPackage Handler and Studio 2019

After having upgraded from Studio 2017 to 2019 I am no longer able to work with StarTransit Packages in Studio. I have installed the plugin "TransitPackage Handler" for Studio 2019 which worked perfectly with Studio 2017. Now, in Studio 2019, I can open the package (defining details such as location, template etc.), but when the package has been opened/created in Studio, the files have not and cannot be prepared for translation (error message under "Convert to translateable format": "Object reference not set to an instance of an object").

I have installed the latest SP1, I have tried to de- and reinstall Studio (and, of course, the plug-in), but to no avail. The file format of the Transit package is .PPF. I have tried importing/opening older Transit packages which worked well under Studio 2017, but that is not possible either. I can, however, still access my older projects which were first imported/opened under Studio 2017, and it is possible to create new return packages for those older projects as well.

Has anybody experienced a similar problem and found a solution? Any help will be appreciated - thanks a lot in advance!