How did you install the new version ?
Can the beta use files and settings from an older version of mIRC? Yes, it can, but...
Like all new versions of mIRC, the beta makes changes to the way some items are stored in the mirc.ini file that are not compatible with older versions of mIRC.
You should not run the beta from the same folder as another version of mIRC as they will interfere with each other.
If you want to use files and settings from an older version of mIRC, you should copy them to a new empty folder and then install the beta in that folder.
So if you just used the new .exe file, it could be a conflict with the mirc.ini file, try to make a clean install in a new empty folder.