After a lot of manual fixing, I managed to get Slackware-current working fine. However, I had to install the man and man-pages from swaret because it wasn't installed earlier.

Now man works fine as a user, but as root, it doesn't display anything. Just a blank page. I installed groff because man seems to require this, but it still doesn't work as root.

It's hard to say what is the problem even when you did lot of manual fixing only known to you. In my experience Slackware does not need such "fixes", just editing configuration files.
I would try to remove subdirs for man cache files (there may be a problem with permissions) and reinstall man and groff package for sanity Concerning the cat subdirs, what permissions do they have ? Btw. they come from aaa_base package.

Those fixes were for other problems including a few dependency issues with other programs and glibc.

man does work fine as user. But when I su to root or log in as root, it just displays an empty screen. Not sure how to fix this, although it's not a major problem.

does strace give you any clues
as to what might be happening
under root but not user?

Maybe you should try what I was suggesting, maybe man tool refuses to display content of a cached file when with improper permissions, but that's only my doubt. I'm running 2 boxes with Slack current and can normaly read man pages as root.

Just another idea concerning possible wrong environment settings. Can you su to root wihtout '-' option to keep users settings and try it again ? Also check PATH, MANPATH and TERM environment variables when logged in as root. May be it has something to do with different setting of locales. Try Stracing a problematic app is also a good way if you know what you are doing.

1. check your PATH (for root)
2. why would you need root privileges to read man pages ?!

Well, sometimes I su to root to do some operation and then require to check up a command. In that situation I use the man command.

Also, now I tried compiling 2.6.16 from Slackware and it doesn't work. I will have to restore grub to point to the earlier kernel before I can try out the suggestions in this thread.

Issue set as root, your MANPATH should be like this:

Strangely problem solved. Maybe the new kernel I compiled (2.6.16) did it, but I'm not sure. Anyway, works now