There is a problem with the security certificate of the proxy server. Error code 18 and 38.

Question

Hi All,
After several hours and a short night of sleep I'm out of ideas and hopefully someone here can help me trying to solve this one. First of all the situation:
Exchange 2013 on a remote location with a CA-certificate.
Outlook 2010 and 2013 on different locations, locally installed and on RDS.
When I open Outlook on my laptop all is fine, no errors, good sync, no problem. But when I open Outlook on our Remote Desktop Servers with Outlook 2013 I'm getting errors like "There is a problem with the security certificate of the proxy server. The
name on the security certificate is invalid or does not match the name of the site. Outlook is unable to connect to this server. (Error code 18)". Opening Outlook 2010 the message is the same, but the error code now is 38.
After this Outlook opens and is working, there's one more error though. After a while an security warning pops up with the message: "Information you exchange with this site cannot be viewed or changed by others. However, there is a problem with the
site's security certificate. * The security certificate was issued by a company you have not chosen to trust. View the certificate to determine whether you want to trust the certifying authority. * The security certificate is valid. * The name on the security
certificate is invalid or does not match the name of the site."
Strangest thing is, it is the certificate of my RDS! It isn't my valid en officially bought certificate from my mailserver. What's going on? I'm out of options, what I've tried so far (in random order):
- restarting mailserver and AD;
- restarting switches;
- restarting routers;
- restarting RDS, AD and all other servers;
- bypassed proxyserver for RDS;
- created a new profile;
- checked recently installed updates;
- checked certificate on mailserver;
- checked RDS on a different location, working fine.
Nothing helped, what can I do next? Please advice.
Regards.

Answer 1

Found a thread that solves half my problem (https://social.technet.microsoft.com/Forums/office/en-US/70d18244-889a-4d95-ac3f-e234672a82b2/there-is-a-problem-with-the-proxy-servers-security-certificate-error-when-starting-outlook?forum=exchangesvrclients).
The first message can be suppressed by adding this to the Exchange config:
set-outlookprovider -Identity EXCH -CertprincipalName msstd:webmail.domain.tld
set-outlookprovider -Identity EXPR -CertprincipalName msstd:webmail.domain.tld
Giving the command get-outlookprovider, gives me empty information regarding the certprinipalname. Filled
this and after recreating the profile or deleting the ost-file I still have the second alert with the local certificate of my RDS.
Not completely where I want to be, any help regarding the second alert is greatly appreciated!