Thanks for the update, much appreciated.
I don't believe I deleted the other two words that seem to be problems. All three were words I typed myself in both English and Arabic, rather than adding the suggested definitions. I deleted "plants, laboratories" and as it made no difference I re-typed the entry from scratch and put it in its original place. When I originally got stuck when the lesson was called "lesson 8" ((before duplicating to produce "copy of lesson 8" and deleting the original) that was when I think I deleted it and it had no effect so I did the duplication. I can't remember if I moved it before deleting or if I deleted first then added it again and tried moving it but I moved "plants, laboratories" from its current position to the end of the list to see if it got past the error message. As it didn't, I moved it back into its original place (as I'm trying to maintain alphabetical order). I didn't move or delete any other words including the other two problem words "workers" and "valuable".
The problem with the course's bug seems to also be affecting my statistics in my pro membership- I can't view this course in the pie-chart of the time spent on courses any more and it's wildly distorted the time spent on other courses- some which I have spent tens of hours on are recorded as having been under an hour each or just a few hours in the last 60 days.
Let me know if I can help in any other way and thanks again for the update.