I have a client that has ~40k documents in a document library and this has been the case for years. Suddenly, two days ago, several flat views just stopped working with the dreaded "The attempted operation is prohibited because it exceeds the list view threshold" error message. I deleted 5k test documents to see if some sort of hidden threshold had been hit, but this didn't change anything.
Something changed in the infrastructure, I believe. I also try to create routine views on other libraries with more than 5000 documents that always worked and don't anymore.
I don't really want to go through the details of each affected views and the possible work arounds, but does anyone know if something changed lately in the SPO infrastructure about the LVT?