PROBLEM setting external link: changedata.php

[SoHo22]

Hi there,
I have a problem with external links on one of my websites. Every time I try to set an external link in the text a popup appears

"you want to open the file changedata.php"

and the page can´t be saved.

What can I do and what could be the reason for this strange behavoir. I work with GetSimple now for several years now but never had such an issue.

PHP version is 5.6, permissions are set to 755 / 644, an index.php file exists in the /admin-folder.

Thank you
SoHo