Mind telling us what the fix (and problem) was? I haven't noticed that behaviour personally, but it might be useful if someone else sees the same thing.

The problem was that when I clicked on quoted post it would open in a new tab to the default plain http address (no redirection to the secure one), although I had HTTPSEverywhere installed and the ruleset in the correct path.

Checking things out I noticed that the ruleset wasn't recognised by HTTPSEverywhere in a consistent manner. So, I closed my browser, purged the ruleset file, made a fresh one, put it in the its subdirectory, and started my browser.
It seems to work properly ever since. :-)

Ruleset additions/modifications do seem to require a browser restart or extension restart in order to become active.