Error when uploading images

Hi!
I’m using NextGen Gallery on a domain and a subdomain (of this domain) (two different WordPress installations).
I can upload images and use the gallery on the subdomain without any problems.
But I can’t upload images on the domain – when doing so I get the error “An unexpected error occured. This is most likely due to a server misconfiguration. Check your PHP error log or ask your hosting provider for assistance.”

These steps I have already done/checked:

– same settings in the plugin (domain and subdomain)
– same server configuration (same server)
– same access permissions on the folders
– same installation settings on the server (I chatted with the provider’s server administrator)
– same PHP-version (I’ve tried 5.6 / 7.0 / 7.1)
– same memory limit (128M)
– no errors in server logs
– no errors in application firewall

– all other plugins deactivated
– all other plugins deinstalled
– NextGen Gallery deinstalled and reinstalled

– upload in WordPress Media Library works without any problems > import images from library into NextGen doesn’t work > doesn’t stop loading and I can’t do anything else (even after deinstalling and reinstalling the plugin the message that images are loading is still there).

So what else can I do?
What is the problem? Why is the upload working on the subdomain installation but not working on the domain installation?

Please help me! Thank you!