Cannot login after 3.7 to 3.8.1 update

Updated 3.7 to 3.8.1 on a working installation. After database update (no errors), I can no longer log into the site using my admin credentials.

I initially got this error:

Error: Form security failed. Please reload the form and check that your cookies are enabled. If you use a proxy server, you must configure Matomo to accept the proxy header that forwards the Host header. Also, check that your Referrer header is sent correctly.

Also, you may force Matomo to use a secure connection: in your config file config/config.ini.php set 
below section 

On subsequent logins, the page just refreshes without any errors. Tried on different browsers and computers, which all exhibit the same issue. Strangely, the IOS app works with those same credentials. I do get an invalid user/pass if I do use wrong credentials.

I do have a reverse proxy (haproxy) as well as a cache layer (varnish) before apache 2.4, with remoteip module enabled. This was working just prior to the update. Debug logs don’t show anything when this occurs.

My [general] section in my config is this:

assume_secure_protocol = 1
trusted_hosts[] = ""
salt = "salt-value"

What should I look at next to troubleshoot this?

I’m experiencing the same problem.
@wuffers Have you solved it in the meantime?

Any other ideas?