Hello my friend!
I dug up G's answer from couple years ago, maybe it will also help for you:
"That typically means EUI did not end up in your Civ 5 DLC folder. Move EUI out of the c:/ directory for that and put it in your alt directory."
i did not understand perfectly.
(im having unable to load texture errors [DCx_diplomacyxx.dds] kind)
My game was working fine before i formatted my pc.
Shouldn i just need to install populi installer as admin, do i need to do something else?