I think I found the origin of the problem.
In my CS50 IDE, I had managed to create the bday.txt with incorrect user permissions: 620 in stead of 664. (So I was the only one who could read the file ...)
After correcting the erroneous permissions, and after repeating the check50 (and submit50) command, the versions on github were still empty.
So in fact, the actual answer to the questions in the opening post were:
- Yes, there was something wrong with my bday.txt file (incorrect user permissions)
- Yes, a glitch in check50 is blocking the uploads of the corrected version of the file.
When creating new files which will have to be submitted for a pset, please DO use the steps that are shown in the lessons.
And if you really want to do it in another way, please DO check the user permissions before doing the check50 and/or submit50 ...