I wonder if a solution has been found for this. The error came up this week during a training session. The computers involved had Multiterm 2019 installed and a freshly downloaded version of the Glossary Converter from the app store. No SDL products had been installed prior to this on these computers.
Not sure if this matters, but the computers where the issue happened had just been used to do an Excel to Multiterm import using the Multiterm Convert procedure. Right after that, the Glossary Converter was installed to demonstrate its advantages and all the computers got the same error. When a conversion was attempted, the resulting file was a tbx, and the Glossary Converter showed tbx instead of sdltb in the "Out" pane of the main app interface.
The one computer where the app worked was mine, where a) Studio 2017 had been installed before, but not Multiterm, b) Multiterm 2019 is installed and c) the computer was not used to do the Multiterm Convert procedure prior to running the Glossary Converter.
The students are puzzled and would love to be able to use the Glossary Converter, so I promised I'd check if a fix has been found.