Most of us have been advised by support to file a bug report. ImportStars.txt, as you described ( one system per line) is SUPPOSED to be working, however with the new visitedstars cache updates it is not.
Not aware of any mechanism to export an old visited stars cache file to a list of systems. If you send data to EDSM, you could compile a list of systems you've sent to EDSM, which should be of help, however, to reiterate, there's no mechanic at this time to import that data to a current game installation. If you aren't on EDSM, you should be if you care about having a good record of your exploration efforts.
You can upload your game journal to EDSM, and it will process them. (if you aren't already participating on EDSM)