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Ok -Here are some things to try
Can you check your admin/groups/permissions and make sure you actually have permission to access the forum blocks?
Check the FAQ section on blank pages and see if there is nothing there that might help.
Finally - when things sometimes appear and sometimes do not often doing a 'repair' on the sessions table does the trick. Go to you sites phpmyadmin, find the XOOPS sessions table and click repair. See if that makes a difference.
If none of this helps It is time to check your debug messages -Turn on inline debug and press 'all' and see what it says - looking for php errors and warnings. Follow the directions in the read before you post below in my signature to insert the debug message so we can see if there is a clue in there.
Good luck