8.12,8.14,nightly all install on linux mint, but 8.15 wont run

I have run arduino 8.12, 8.14--it runs fine on my linux mint machine. However, updating to 8.15 gives an error when trying to run:
arduino-1.8.15/java/bin/java: cannot execute binary file: Exec format error

I re-downloaded x86-64 bit several times, 8.15 always fails, but 8.14 downloads and runs fine. I uninstalled, re-downloaded 8.14 to see if this was a pilot error, and re-installed. 8.14 runs fine. Then I uninstalled, redownloaded 8.15, installed--but still get that error. I repeated the same process with the nightly build. It works fine. I checked for forum entries with the same problem, but I dont see anyone else having this problem.
What's different about 8.15 that will give the exec format error?

It's very unexpected that there would have been such a difference between these two versions because Arduino IDE 1.8.15 was released only a day after 1.8.14 when we discovered that a new feature would have an impact on some popular 3rd party boards platforms with misconfigured libraries. We decided to temporarily revert that feature and make a release.

You can see that the only difference to the Arduino IDE code base was to change the arduino-builder dependency version from 1.6.0 to 1.6.1:
and the only change to arduino-builder the update of its Arduino CLI dependency from e64bd910f0f4 to d710b642ef79:
And finally we get to the actual code changes, in Arduino CLI:
you can see that a few other changes did make it in to the 1.8.15. Nothing jumps out at me as significant though.

This error message would make me look to whether the incorrect variant of the IDE is being used (e.g., Linux ARM instead of Linux). It sounds like you have considered this already, but definitely worth a triple check.

I finally got it to work--I wiped all arduino directories, (before, I just wiped the 8.15 directory). I repeated all the steps, checking the download names making sure it was not the ARM or 32 bit version downloaded, and now it appears to be working. Thanks for looking at this!

You're welcome. I'm glad to hear it's working now. Enjoy!