Helped by myself. Copied the date part in all language files from the free version and modified all entries with "DATE_FORMAT_ ..."
E.g.: changed "DATE_FORMAT_NORMAL" to "DATE_FORMAT_LC3".

Also copied the date part in defines.php from the free version (around line 74)

I have tried the free version of Book It. Great Component - everything is fine. Only the Credit Link...
So I decided to buy the Book It 2 Paid Version.

After installing I got a 500 error which said:
"DateTime::__construct() [datetime.--construct]: Failed to parse time string (FrPMGMTE_Juni+0000RJunPMGMT_0+0000513) at position 0 (F): The timezone could not be found in the database"

I have tried to uninstall and reinstall, also I realised all here mentioned solutions. Nothing works.

Do you have any kind of a solution.