Registering in the shop went smoothly until I tried to follow the activation link. Then I got an error message about an invalid email address format. That seemed quite bizarre at first, as the shop had just successfully sent me an email. Then I noticed that a part of the activation link looked similar to an email address, but it wasn't my address. And yet some parts of it were eerily familiar.

It turned out that it was the email address I had given to the shop, only it had been obfuscated with rot-13. That address contained a plus sign, and the plus sign was not percent-encoded in the activation link, and unencoded plus signs have special meanings in some places in a URI. Replacing the plus with "%2B" produced a working activation URI.

So the shop software should be fixed to encode its URIs correctly, but until then, if anyone else has the same problem, try this trick.

That obfuscation seems entirely pointless by the way. Not only is rot-13 worthless, but the un-obfuscated address is included in the email anyway.