Save Online not working in MuseScore 3.5 or 3.6 on macOS High Sierra

A friend is having trouble logging in to MuseScore and uploading scores via File > Save Online. The problem randomly started in MuseScore 3.5 a few months back and updating to 3.6 hasn't fixed it.

When she tries to save online, the login page appears and she is able to type in her username and password, but when she clicks "Log in" all that happens is the login dialog briefly flashes "Welcome to MuseScore" (or something similar) and then goes white. The login dialog never disappears and the upload dialog never appears.

I sent her my 'cred.dat' file so she could try with the new --save-online option on the command line. This was the result:

The interesting part of the message:

skipping hostname of len 0
skipping hostname of len 0
"Error while getting user info:  ()".

My friend is running macOS High Sierra, which is quite old now (released 2017) and is no longer updated by Apple. However, it's the last version of macOS that is supported on her machine, so she can't upgrade without buying a new computer.

I wondered if the problem might be caused by expired SSL certificates, but she says that uploading via the browser (Safari) is working OK.