Hey there,
I am really sorry for the delay of this reply as it is a weekend for us here.
Perhaps you can provide us one of the user login account which having this issue so I can better have a check this on your duplicated site?
Because i did tried login as this
temp user, then i upload 11MB file in this duplicated site, it seems i can able to upload successfully without show any warning, you can check my screenshot here :
http://take.ms/2ZAjQ (that zip file i download from Joomla here
https://downloads.joomla.org/)
This
temp user is under this profile type
'Mitglied SNETZ - Sozialwesen' and this maximum upload file size you set to 50MB (
http://take.ms/lHzYy), then the system will allow this user upload the file size below 50MB.
And i checked other profile types, it seems like it set maximum upload file size only 8MB.
Perhaps you can re-check again.
Keep us update if the issue still persists and provide us that user login access.
By the way, it seems like you provided us that FTP login credentials no longer work.