Cannot update extension list on TYPO3 7.6 and 6.2

What was the cause of the issue?

We're still running on TYPO3 6.2 (ELTS) and 7.6, and updating the extension list in the extension manager works with neither version.

This is the error message that pops up:

Could not retrieve extension hash file from remote server.

The extension manager states that the last update was 5 days ago (we run a daily extension update through the scheduler module), which could coincide with the TER relaunch.

Steps to reproduce

Go to extension manager module, update extension list, admire error

Output of error log

None