Here is a good discussion of this:
http://tobaccoanalysis.blogspot.com/2013/06/german-cancer-research-center-corrects.html
I just skimmed through the article again to refresh my memory. It seems that there was one case study where a user of electronic cigarettes was diagnosed with lipoid pneumonia. However, there were other factors that could have contributed to the pneumonia. They never proved that the electronic cigarette was the cause. A case study is just the study of one or a few cases, and generally these aren't taken as very strong scientific evidence. There are no control groups, just a write up of patient histories, symptoms, outcome, etc... In this particular case, there was nothing much to link the ecig to the pneumonia. If someone had a case study of 3-4 patients who used ecigs and developed pneumonia, that would be a little stronger. Also, a case study that wants to link a cause and effect is stronger if it proposes a plausible mechanism of action. This case study proposed it was oils in the liquid that caused the pneumonia, but ecig liquids do not usually contain oil, so the biological mechanism these researchers used to link the two events is implausible.