The same has been occurring for me as well. To make sure it was no other addon, I've deleted my WTF and Interface folders (not before making a backup) and fresh installed RealUI.
The bug only happens when I mouse over either a player or NPC. Tries to asses the iLvl and apparently fails to do so accurately.
Code:
24x FreebTipiLvl\ilvl.lua:81: attempt to perform arithmetic on local 'ilvl' (a nil value)
FreebTipiLvl\ilvl.lua:81: in function <FreebTipiLvl\ilvl.lua:63>
FreebTipiLvl\ilvl.lua:91: in function 'callback'
...e\FreebTipiLvl\libs\LibInspect\LibInspect-6.lua:467: in function 'RunHooks'
...e\FreebTipiLvl\libs\LibInspect\LibInspect-6.lua:337: in function 'InspectReady'
...e\FreebTipiLvl\libs\LibInspect\LibInspect-6.lua:267: in function 'SafeRequestItems'
...e\FreebTipiLvl\libs\LibInspect\LibInspect-6.lua:216: in function 'RequestData'
FreebTipiLvl\ilvl.lua:97: in function <FreebTipiLvl\ilvl.lua:93>
[C]:: ?
[C]:: ?
[C]:: in function 'SetUnit'
FrameXML\UnitFrame.lua:465: in function 'UnitFrame_UpdateTooltip'
FrameXML\UnitFrame.lua:451: in function <FrameXML\UnitFrame.lua:438>
Grid2\GridFrame.lua:299: in function 'OnFrameEnter'
Grid2\GridFrame.lua:92: in function <Grid2\GridFrame.lua:91>
[C]:: ?
FrameXML\SecureHandlers.lua:264: in function <FrameXML\SecureHandlers.lua:261>
[C]:: ?
FrameXML\SecureHandlers.lua:320: in function <FrameXML\SecureHandlers.lua:304>
(tail call): ?
Along with my reply is the zip file (nibRealUI_Init.lua)
Hope it helps solves the issue.