I have a system (BridgeSystem.tex) with several indexes and during the process of adding a new symbol glossary and refactoring a lot of code, one of them went missing. Its .idx file looked structurally identical to the .idx files of the others, but its .ilg file contained the above error for every line and consequently the .ind file size was 0K.
Initially I expected the problem would be related to the processing order of hyperref, glossaries-extra, makeindex and makeglossaries, but that led me nowhere.
There's very little on Google about this.
No MWE I'm afraid - the whole system is complex. But can anyone help?