I am experiencing this same issue when upgrading from 4.5.3 ultimately to 5.2.3, but it begins to happen at 4.6.2. I am working on a test box, to keep my production server running, and as soon as I upgrade to 4.6.2, the ability to login as a superuser reports the error. If I perform a clean install of any version of dnn to the test box, I don't have a problem. If I upgrade to anything past 4.6.1 (only tried 4.9.5 per other forum suggestions of stable versions), the issue is present. I have restored to 4.5.3 and then removed all 3rd party skins and 3rd party modules, verified the time zone settings are in the host account profile, and several other suggestions found throughout the formus, but I cannot get past the error, and I am not sure what to look for. There are quite a few files that look at the 'issuperuser' field, but not a programmer by trade, I can't say that I know what to look for in any of these files.