This happened to me at one stage after a M/B failure which required a new OS installation along with most of the software on the C drive.
I eventually discovered that the most recent version of the anti-virus / internet security program (CA) was deleting all previous cookies on each fresh start-up. Apparently it was the hidden default setting - goodness knows why
.
I disabled this 'feature', and from that point on, logging in to PPRuNe again became a seemless process as it was before the other troubles..
FWIW,
FOR