Lightroom Export And Client Email Delivery Suddenly Not Working

Recently, I can no longer export images from Lightroom to the galleries. An error message appears. Even when I try to send the galleries to clients, they no longer receive any email. What could be the problem here

What is that error message that appears?

For emails, Please see: Sytist Manual > Common Issues > Emails Not Sending or Going Into Spam Folders

Unable to upload because Lightroom cannot determine whether the target destination exists.

This is the error message from Lightroom. The customer emails are sent out without any error messages, but the customers are not receiving them.

Perhaps your FTP credentials have changed and you need to update them in Lightroom? That issue will be between Lightroom and your server.

“The hosting provider migrated the mailboxes to Microsoft 365. I have now adjusted everything, but it still doesn’t work. The host says they can’t do anything more at this point. Could it be that the gallery is not compatible with this setup?”

It's a direct FTP connection from lightroom to your server's sy-upload folder. Once there, sytist can process the files into galleries from the folder on sy-upload.  

The lr plugin needs your FTP credentials to work. If you know they are correct, then it could be a plugin problem - it's a very old plugin and I don't think it's been updated in a long time