sorry for the inconveniences.
this error has happened other times and I don’t know why it happens. It’s an error in the file clients.php when the theme is uploading. The code in this file breaks. Can you disable the theme? If you can’t access to the admin panel, you can remove the theme from the FTP.
When this is done, you can create a new user for me in your admin panel. In that way I can change the code to fix the problem. I have done this with other users and there were no problems. You can trust me. I only want to help you.