Version 12.6 has only been out for a short period so there are likely to be bugs.
I'm not positive but you might be able to run 12.5 with the 12.6 license file. You might be better off migrating to 12.5 and waiting until a maintenance release of 12.6 stabilizes it before migrating to the latest version.
Definitely report your problems through the support area so that they can get fixed. This is only the end user forum.