A family of Microsoft word processing software products for creating web, email, and print documents.
Thank you all.
The Mendeley Support Center told me that this wasn't a plug in issue.
In the end I managed to solve my problem:
Although I did look for the presence of "XE" (index entries), like mentioned in one of linked suggestions, I wrongly assumed that they would show by pressing alt+F9 (like all the code and text behind each citation). Instead they only show if the option of showing special characters is on (i.e. the end-of-line).
I must have inadvertently created them while attempting the creation of the summary (very similar, but different from "index")
Once I removed all the "index entries" by means of physically removing the entire content between graphs (graphs included)
the plugin started working again as there was no "indexed area" anymore.