I could reproduce the issue when TLS policy is set to “2020-05-10 no TLS 1.2”.
After setting the policy to “2018-10-01” I was able to login to Horde mail again.
Seems PHP 5.4 does not support >TLS 1.0
I tried to use PHP 5.6 but no success so far…
https://lists.horde.org/archives/imp/Week-of-Mon-20151207/057064.html