Still installing FSX and came to the stage of adding Rex Essential+Overdrive which was purchased in 2013 from Flight Sim Store. When it came to validation it failed and came up with a message unable to validate.
After a bit of searching found out that the company has folded, gone bankrupt!
Have now emailed REX in the hope that they might be able to assist but any-one with add-ons that require validation from the store I would suggest you contact the developers directly if needing to re-install any software purchased from FSS that require validation.
Regards
Nigel.
I used to be an optimist but with age I am now a grumpy old pessimist.
I'm pretty certain that I saw Rex offering advice on what to do about this, Nigel. Have a look on their website and forum. You can be sure Rex will sort you out, as their customer support is first class.