We are once again having this problem with the latest version, 2.6.10. In a Paratext class of 15, it happened for (at least) 2 students. For one, we tried a few more times, and it eventually worked. For the other, it continues to give the error, which is slightly different than the original error:
File "ptxprint.py", line 384, in <module>
'NoneType' object has no attribute 'prjid'
In our case, we are always trying to change the UI language from English to French.