transfilter.dll disappears and won’t load in IIS

I just had the most frustrating experience. I am in the midst of upgrading OAM 7.x to 10.1.4.3. I had all the components upgraded to 10.1.4.0.1 so I got backups taken.

Afterwards one of the webpass instances did not work. Upon further inspection transfilter.dll was actually removed form the file system.  Strange, but no problem, i grabbed a copy from another 10.1.4.0.1 webpass.

The transfilter.dll still didn't get loaded by IIS and it still did not work. I ended up trying numerous different things. I added and re-added transfilter.dll to the Default Web Site multiple times. I uninstalled 10.1.4.0.1 and re-upgrade; still go the same behaviour. I uninstalled webpass alltogether (purged registry, etc) and installed 10.1.4.0.1 instead of upgrading an old one; still got the same behaviour.

I had given up and was about to head home when i decided to try adding transfilter.dll at "Web Sites" instead of "Default Web Site". Low and behold it took and loaded. Then I tried moving it back to "Default Web Site" and it worked there too. I was back!  Frustrated, but back.

I have no idea what caused transfilter.dll to disappear in the first place. I have no idea why it would not load in IIS after it was replaced. I have no idea why moving it in IIS fixed the problem. I am thankful though and if this strange occurence ever recurs I will have a strategy.