After trying all the solutions described, this what I found, and how I made it work (kudos to SherylHohman for her comment in Animesh Chandra's answer for spurring me on).
For some reason I thought, 'just click on the whitespace above "always use this app to open .log files"'
and, lo and behold...
and things go back to working the way they used to.
---- edit ----
and then I find I am facing the same issue as Daniel Hári. The file reverts to opening using windows notepad (not much use, since it's an rsyncd.log file, with unix-style line endings).
so -- run regedit as administrator, and make these changes... (exporting them as .reg files results in hex output for the "ItemName". This format seems more readable). Note that the .log extension had no sub-keys on my example machine, so I did not edit that key. There are other extensions with sub-keys that may be of interest to you. I found them by searching the registry for the string txtfile .
The change seems to have become more sticky (all bets are off following the next feature update).