Today I decided to try using $pages->findIDs() for the first time. It tried it in a template using the same selector that is already successfully working in a $pages->find() request. I have also atttempted to use it with a very simple selector eg. $pages->findIDs("template=post"). I have had to remove it from the template since it always comes up with following error:
The site is using Processwire v3.0.42.