Sign In Required - unable to validate serial number

Hi all,

I had a free trial version of Adobe Acrobat. I then bought Adobe Acrobat Pro standalone offline version. After installing it, I got the message 'sign in required' then when I clicked on the button, it said unable to validate serial number. For the first few days, every time I chose 'quit', I was able to open pdfs and continue regardless, but today, it wouldn't open any pdf and just kept opening the 'sign in' window.

I've already tried contacting Adobe help. Their first suggestion was to take control of my computer and sort it out remotely. I obviously refused. Their next suggestion was to uninstall the software and reinstall and delete a line from the hostfile. I did that, but it didn't work.

I tried to cancel the free trial version I had, but there is no option to cancel in my account, and the helpdesk said I didn't have any subscriptions, so I don't know what else to do. I can't find an email address to write to send screenshots of my account which shows I've bought the product.

Any other suggestions please??