You're not alone.  I just tried to convert a 2019 TT (online ".tax2019") file to 2019 ProSeries.  It fails with "unknown error."  I used to see this if the TT file was password protected,  but this is not the case.

I already pay too much for ProSeries licenses.  I'm not going to buy TT also just so I can import a file that ProSeries is already supposed to import.