For who also has this 'problem'.
Here the solution from the theme developers:
Thank you for your purchase!
Yes, I am aware of that, some servers do not like the uppercase naming of the files.
To fix this you need in the language folder, the en-GB folder, to rename en-GB.tpl[YOUR-TEMPLATE-NAME] to en-GB.tpl_[your-template-name]. (Use no brackets, just your template name!)
And also in the template folder - your-template-name - templateDetails.xml, the same renaming to the language file with lowercase.