@jonasfa Did that happen right after creating the staging site or did you make any changes to the staging site?
Please enable the WordPress debug.log file on the staging site!
WordPress will then write the exact error message and the name of the plugin or theme causing the error to a file called debug.log.
Try opening the staging site again and send us the debug.log file.
It should contain the exact error code preventing your site from being created. You can use the form on this page.
Please read how to enable the debug log and where to find it:
https://wp-staging.com/docs/enable-wordpress-debug-log-mode/
I guess that one of the plugins throws a fatal error. In that case, you have to remove or rename the plugin causing the error to get access to the staging site.