Update… I received an e-mail and I have send login details of the WP installation. Catch-themes did research and then I received the following text in an e-mail:
“The issue is caused by the server where you are hosting your site. There is no any issue with the theme. Please contact your server regarding the issue.”
I have asked for more details about the “server problem”, but I did not get any unfortunately. Well, thanks for looking at the problem.