We are using a hosted webserver (by all-inkl) with wordpress, now one of our cusomers sent a screenshot of this error message, but on all our PCs it works perfectly fine, with the same browsers, he was using IE 11 and Google Chrome V 74.0.3729.169
Looks like this customer uses a firewall or Anti-virus software with a SSL inspection. So that software creates an own certificate and is a Man in the middle.
But that's a problem of that customer, not of your website.
Can you share the screenshot? With the certificate informations?
There it should be visible if it is a Anti-Virus-Software certificate.