while working to resolve a problem in with someone's site, I finally got the login issue fixed.
This is a clean set of 2.1.2 files, no mods, no themes installed yet
the php error log is loading up with the following error.
[Thu Jul 28 15:38:28.191703 2022] [lsapi:warn] [pid 20163:tid 139962892437248] [client .....:42092] [host www.domain.nl] Backend log: PHP Warning: preg_split(): No ending delimiter '/' found in /...directories.../Sources/QueryString.php on line 442\n
that line is
list ($cidr_network, $cidr_subnetmask) = preg_split('/', $cidr_address);
What is set in the proxy configuration as allowed proxy servers?
(I mean, yes, this is clearly a bug. That's not what you pass to preg_split, it's not a valid regex, but I'd like to know what's been passed in so we can figure out how to change it.)
Auto-detect Proxy IP Header = ON
Reverse Proxy Servers IPs
192.168.1.28, 173.245.48.0/20, 103.21.244.0/22, 103.22.200.0/22, 103.31.4.0/22, 141.101.64.0/18, 108.162.192.0/18, 190.93.240.0/20, 188.114.96.0/20, 197.234.240.0/22, 198.41.128.0/17, 162.158.0.0/15, 104.16.0.0/13, 104.24.0.0/14, 172.64.0.0/13, 131.0.72.0/22
OK, I think you can get away with changing the preg_split to explode since it's not doing anything fancy with the split.
Probably should verify that actually there is a / before splitting...