I was just entering a new source. While in the middle of it, something happened (I might have hit a wrong key, but am not sure), and the Source Definition screen disappeared.
Right after this, when looking for that particular source in the "reduced" Master Source List (MSL), I could not find it anymore. The respective number (source no. 529) was not there so I assumed it had somehow not been saved. I then tried to start over, but was only offered the next available source number, i.e. 530. The MSL looked as follows: ...527, 528, 530. I attempted to renumber this new source 530 to the previous 529 but received the message that 529 already existed. However, it showed up nowhere in the MSL.
After trying out various futile approaches, I finally expanded the MSL via the "More" button, and much to my surprise there was my invisible source 529. When switching back to the reduced view of the MSL, the source was again invisible. All the usual (file integrity validation, reindexing, restarting, etc.) did not help.
Any ideas are welcome. Thanks in advance!