Thanks very very much Aadesh, that have helped!
I have a similar problem to the original post. I've just installed Digital Edition 4.5 on a Windows 10 system, and I'm getting the "Error: Check Activation!" message.
However, when try to de-authorize as suggested I get the following error:
"Unable to Erase Authorization. Please try again after some time."
I've been trying on and off for several days now and I always get this message.
I don't know if it's relevant, but when I installed DE 4.5 it said it had detect a previous version and was importing my ebooks and settings. I haven't used Digital Editions for a while so the previous version must have been several years old, and would have been installed before I ungraded from Windows 7 to Windows 10.
Any help appreciated.