Sorry for not highlighting this earlier (my upgrade hasn’t come through yet) but for anyone upgrading to 4.6.0.15, I strongly recommend removing ESR 91 before upgrading, following the steps in the “Restore ESR 78” instructions. In particular, as @direc85 highlights, you should remove the symlink and restore your ESR 78 profile directory.
I’m pleased to see that Jolla has included some libhybris changes in 4.6.0.15 that will help avoid some types of browser crash, so if you reinstall ESR 91 again after the upgrade you should get a better experience.