I suspect something is corrupt - did you recently upgrade your client? Did you install 9.0.1 over the top of 8.5.3? Or Uninstall 8.5.3 first?
You might try closing your client, then rename cache.ndk and desktop8.ndk. Open your client and see if Followup is back.
If not, close your mail file - at the server console type: load convert -u mail\taylor.nsf * mail9.ntf
- where 'taylor.nsf' is the name of your mail file, and 'mail9.ntf' is the name of your mail template. This will refresh your mailfile design & all your folders.
If that doesn't help - at the console type:
load fixup mail\taylor.nsf -j -f
load compact mail\taylor.nsf -c -i
load updall mail\taylor.nsf -r -x
again, where 'taylor.nsf' is the name of your mailfile.
You said you already deleted the workspace folder?