Assuming all your configuration is correct I suggest reaching out to PayPal as sounds like a problem on their edit. I recommend double checking the gateway configuration in CBSubs though to be sure you've supplied everything exactly as instructed. Below is my recommendations

PayPal Business (email or secure merchant ID): Suggest Merchant ID here otherwise use the email address marked as PRIMARY in your PayPal account

PayPal Main receiver account (email) (if Business is different from your main account): This should be the email address marked as PRIMARY in your PayPal account

Your PayPal Identity Token: Ensure you copied this from your PayPal account completely as sometimes double-clicking the input to select its value at PayPal only selects up to a hard break like a dash or underscore.

The PayPal API credentials are optional, but I recommend also setting them.

There's not really anything we can do about errors on PayPal's end since this is not happening in CBSubs itself. Have you tried contacting PayPal and seeing what they can find on their end?

Does you provide any kind of support for take a look of this issue and give us a better overview of the problem?

We do install reviews when it seams necessary. Only thing I can do about this would be to just look over your configuration, but since this is happening on PayPal's end there's no tools for us to use that can help find what's causing this.

Typically that error means the configuration is wrong and should be corrected as instructed above or there's a problem in your PayPal account itself. This can happen if "Encrypted website payments" is set to "On" within PayPal > Settings > Seller Tools > Website preferences. Try setting it to "Off" if it's set to "On" and see if that helps.