Ah, it's when you open the menu? This is due to a limitation in GMod.
Anyways, it's an easy enough fix: Open up ulx_menu_base.lua and edit line 35 and 36 to be:
gCurKey[ i ] = gKeyOffset
gCurRectKey[ i ] = gKeyOffset
Then edit line 60 and 61 to be:
if gCurKey[ userid ] > gMaxOptions * 2 + 6 + gKeyOffset then -- Shouldn't be anymore than two menus blending in animation ( the extra six is from the title, next, and back text )
gCurKey[ userid ] = gKeyOffset
And lines 140 and 141 to be:
if gCurRectKey[ userid ] > 2 + gKeyOffset then -- Only two menus should be blending together
gCurRectKey[ userid ] = gKeyOffset + 1
Lastly, add this line to the start of the file:
gKeyOffset = 600
There you go
If you have further conflicts just edit the offset.