Drop down menus are blank

Libre Office 5 came with UBUNTU upgrade to V16.04 has this issue is with just one ODT file. Same file works OK in UBUNTU V12.04 and in WIN XP

However, in UBUNTU V16.04 and using Libre Office V5 for most of the time (I said most) I cannot get any drop down Menus. Just grey boxes - no sign of any text or choices. Ctrl+S will save my file and all the entries can be made normally.
Very occasionally the condition clears and everything is normal again. For no reason I know of!

I tried pasting the contents into a NEW doc. Same result. Usually slow to load. Swapped memory allocations in OPTIONS to 200 Mbyte but still no good. Still the same. It’s nuts please help me. Urgent!

Sincerely and thanks
Berry