I had exactly the same problem when I upgraded from 5.3 to 5.4.
The problem was that the job PGM-QLWISVR was ending abnormally.
When I looked at the joblog for the user that started it (QTMHHTTP), it failed because the user profile did not have the required authority to the object (QLWISVR).
I changed the object to include QTMHHTTP user with *USE authority and started up the HTTP server *ADMIN instance again. This job now runs and the ADMIN page is now displayed correctly.
I hope this cures your problem.
I had the same problem but it turned out to be a PTF installation priority issue:
If Error 500 appears on selection of the Web Administration for i5OS tab in the browser and the QLWISVR jobs are dying, and if 5722JV1 Options 7 & 8 were installed, but at a different time than 5722DG1 (and all PTF’s applied), THEN
Go back and reapply ALL group PTF’s for *ALL products. It appears there are interdependencies in the applications and PTF’s that require everything to be in place at the appropriate time when the PTF’s run. Having 5722DG1 installed and patched and then installing and patching Options 7 and 8 for 5722JV1 was not sufficient in my case. It required reloading and reapplying all of the group PTF’s at once. After reloading and reapplying the group level PTF’s, the ADMIN2 QLWISVR job fires right up and Web Administration seems to work just fine.