For more than a year now, I get occasional customers who elect to pay me using paypal, but when they click the link on my website which 99% of the time works fine, they instead report to me that paypal shows them this:-

Access Denied
You don't have permission to access "http://www.paypal.com/cgi-bin/webscr" on this server.
Reference #18.c5595068.1537455748.868cf0a

That is not my server showing that message - it's something purporting to be paypal (it says http: in the error, but as far as I know, it's an https: page).

These are intermittent problems, and it's my customers, not me, who suffer them, so I am unable to reproduce them - I only know when my customers are kind enough to tell me - the problem is probably 10x larger (most customers probably don't report this).

Has anyone else seen this? How would I get it fixed?