Firebreath, if ulx doesn't work right when in DarkRP, but ULX works fine when in Sandbox, then you need to ask DarkRP authors for assistance telling them it conflicts with ULX. ULX is being broken by DarkRP, not the other way around.
There have been tens if not hundreds of posts on this forum regarding ULX breaking when DarkRP is used. There have been tens of posts where exploits in DarkRP or it's plugins were used against servers, some of them running ULX and getting ULX blamed.
Without further errors from your server or game console reported about ULX or ULib files, we really can't assist you further other than recommending you only use ULX during sandbox gamemode.
One other tip I can think of, many of the other people reporting DarkRP problems found a 'unauthed player' error from ULib in their consoles.
It seems to fix that, removing NewAdmin from their local machine and server corrected it. (It seems NewAdmin comes with DarkRP, I've never investigated this)