Problem logging in - "check cookie settings"

[tomos]

Having a problem logging in on my laptop:
# Firefox 39.1.0 ESR /
# I use LastPass (dont actually know my dc password)
# I installed FF ESR last week - logging in to dc worked until last-night.

I get the message:
"You were unable to login. Please check your cookie settings."

Cookie settings are:
# Accept cookies (I even actively added dc to allow list)
# *Never* accept third party cookies
# Keep until they expire

When I search cookies, there are 5 or 6 from donationcoder.
I have been able to login to a couple of other forums.
Using uBlock for ads - but makes no difference if I disable.

Any ideas?