"Processing view failed" error on report

Content

I have a report where I have a column which is a concatenation of text and fields and one if statement, so essentially running a bit like a merge. It was all running smoothly until I made 4 small changes, each of which changed a part of the concatenated text to an if statement and each returning text in the then and else sections.

However in doing this I now get "Processing view failed" when I try to run. If I change the column back to the previous version it works again, so it's definitely one of those changes, but can someone tell me what it even means by "Processing view failed" to help give me a chance of working out what it doesn't like about it?!