This is not a permanent fix, but a useful workaround, in case you are not already familiar with it:
these packages are essentially compressed packages, and therefore you can open them to extract the content manually if Studio fails to import it for some reason.
To do that, rename the extension (.sdlppx) to .zip (and accept the warning message) and extract it as you normally would a zip package. You will be presented with the translatable file, and all the other relevant resources sent in the package (project TM, termbase). Then you can open the file in Studio, add the memory, termbase and apply the necessary project settings before starting the translation.
Please note that with this workaround you will not be able to create a return package when you're ready - in that case send the translated sdlxliff, or even better, zip up the project folder and send it to your client.
Hope this helps.
Greta Magyar - Customer Success Representative - SDL