Paypal Message 3005 errors in sandbox

I’m getting the Message 3005 error in the sandbox with encrypted paypal buttons (not with non-encrypted equivalents) no matter how simple the button.

Looking back over the forums I can’t find anything really definitive about whether there was a problem that can be fixed at the website end; can anyone point me to a checklist of things to look through?

Urgh, this is still happening.

I’m on latest wordpress, latest s2member, testing with either a Website Payments Pro or Business sandbox user. I’ve even tried a whole new sandbox account. My purchaser and buyer accounts are both UK accounts. I’m doing the simplest (default) level 1 button, just set to GBP, and as far as I can see from the S2 log and the generated HTML, an encrypyted button is being created successfully.

The error occurs after I log into my overall sandbox account, but before my purchaser account even gets to log in, which suggests it’s at the point of button decryption that the problem occurs. I can get past this stage with an unencrypted button.

I appreciate that this is likely a Paypal problem (I’ve experienced sandbox-related pain before) and support doesn’t hang out in this forum or answer users of the free project, so I was wondering if anyone who has a similar setup to me could test this in their sandbox account? Is there something I’m doing here that will cause it to not work?

This first project with S2Member is a test case for a larger project in the future, and while I could test against a live account with small purchases on this project, I’m really uncomfortable with doing that in the future.

Does anyone have any ideas that would give me a different angle of approach here?