Addon error.
 

Error of some addon.

Looks like Arkadius Trade Tools or Master Merchant or similar addons adding item price data to the tooltip.
Try to check if you got any pricing addon enabled and disable it, to check which one it is.

If you found ot which one it is write to that addon's comments please and add the information about the item you moved the mouse over -> Maybe show them the itemlink data by right clicking that item -> choose "To chat" from the context menu. Then mark all chat box text of the item and press CTRL+C to copy the itemlink to the clipboard.
With CTRL+V you are able to paste that itemlink (looks like |h0:... or |h1:...) to the addon comments.

This sometimes appears when I just go into the game or reboot to the twink, and not when I click on some item. I will try to disable trade add-ons. (Sorry, I do not speak English, but I only know Russian and write through a translator).

Do you use AUI as well?
I have read in the ATT addon comments that the bug is coming due to AUI + ATT enabled.

Yes, I use these addons. Well ... I'll wait, maybe they will fix it:)