Outlook Anywhere keeps asking for credentials

We have switched our SBS2011 (Exchange 2010) from a self-issued certificate to an SHA-2 one from GoDaddy.

Then we want to connect to it using Outlook Anywhere, basic authentication. We have some users using Macs, and to avoid NTLM problems with Macs we chose basic.

Now Macs connect fine, no problem.
PCs connect as well and session is stable, but every 3-4 minutes a prompt for credentials keeps coming up (on PCs only, not on Macs).

Many sites on the web recommend the

Set-OutlookProvider EXPR –CertPrincipalName “msstd:site.com”

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command, which we did, but no fix.

We noticed in Outlook settings "Connect using SSL only" is checked and grayed out - while our new certificate is SHA-2 (?)
Please advise (what did we do wrong?)