It would be great if you could provide the community with some specifics like which mod(pack) you are using. And the versions involved etc.
Besides, it helps to search these forums/google for a possible answer instead of just dumping the question in here.
There seems to be an answer to you problem:
(did not use this answer, but it seems better then the second one I used)
http://forum.feed-the-beast.com/threads/unable-to-light-a-nether-portal.23499/#post-302200
(bottom post in my case)
Good luck
I personally did something else which helped:
I found this post before the above one, which seems better.
http://forum.feed-the-beast.com/threads/1-5-2-0-7-modpack-bug-reports.25010/page-13#post-397613
This post lead me to my <MC server folder>\config\railcraft\railcraft.cfg file.
In here the only value I found which came sort-of closest to the value mentioned in the post above was: (I can't make 'code' bold, but I meant the "I:maxsize=9" part.)
Code:
####################
# irontank
####################
irontank {
#
# Change to 'allow.stacking=false' to disable the stacking of Iron Tanks
#
B:allow.stacking=true
#
# Allows you to set the max Iron Tank base dimension, valid values are 3, 5, 7, and 9
#
I:maxsize=9
}
However ridiculous it seemed, I changes this value to 0, restarted the server and BOOM. My portal lit up like a zombie at noon.
My portal works now and keeps working even when I changed this value back to the original '9'.
I did
not test the creation of a second portal after I changes the value back.
Disclaimer: I do not have any knowledge of config files and mod making, so use this at you own risk. (although I can't think of any real bad things that could happen...)