Provider threw exception in DoAuthorize.
Re: Provider threw exception in DoAuthorize.
It seems that some how PayPal SSL certificate is getting failed in validation check. The reason I could think for this problem is that most probably your hosting provider updated their .NET SSL security policy that's why PayPal SSL cert is not passing new settings resulting an exception. Ask your hosting provider for this scenario. Secondly you may also ask your hosting provider to include PayPal SSL in their trusted certificate list hopefully this will also fix the problem.
Re: Provider threw exception in DoAuthorize.
Yes definitely this is the cause. Its something not supposed to be fixed by AbleCommerce code. Suggested fix or fix configurations are supposed to be applied on your server. I have gone through link provided by you. PayPal representative are saying that changes will effect PHP SOAP SDK v4.3.58. I think this effected .NET as well. Better contact PayPal and tell them that you are having problem on .NET C# due to this change and then apply their suggested fix on your server either by asking your hosting provider or your self.
Re: Provider threw exception in DoAuthorize.
Sounds Great