I'm having the same issues. Here is what i just sent to Paul if it helps anyone..
"I am now able to replicate the problem over and over. I do have cax files for 2015 I have been using for a long time as well.
Here is how I replicate the issue over and over.
-Open 7.5
-open lml tune file and it will say “Os no supported” or whatever
-close software
-reopen software and it will then open the file without a error automatically
So when 7.5 boots up and automatically opens its last file, It will open with no issue. The issue occurs when I try to “open” the tune within the software.
Hope this helps."
Also- my problem is not limited to files that ONLY have .cax...it happens on others as well.