Whilst I wait for a response from my hosting company, I clutch at straws.
I therefore refer you to this thread whose issue you recently resolved:
Tht also involved an update from PHP 5 to PHP 7, and there was -- if you recall -- an issue with a default link that inadvertently declared an unworkable URL and file path -- for File Manager -- by appending an old path with a directory and file name to a new one, their concatentation being an attempt to point to a location that did not exist. You quickly -- in that other thread -- corrected the issue by amending the default.
Is it at all possible that given that this is also effectively involving an update from PHP 5 or 6 to 7.xx some FILE declaration is similarly appending both an old and a new directive, and such as there might be in the test email you uploaded?
I apologise sincerely if that makes no sense, and can only ask for your forgiveness for in that case suggesting it.
All we can then do is await further details from the hosting company.
Thank you for your kindness, and I again apologise if the above that possibly file names and etc are being inadvertently concatenated is an absurd and senseless notion,