"Application popup: : "

"Application popup: : " Event ID: 26 can be anything. Basically means a nameless application has thrown a nameless error. After a while the worker application pool process w3wp.exe would throw other events on systems tab like:

"A process serving application pool 'AppPool11' terminated unexpectedly. The process id was '1932'. The process exit code was '0x1'."

"Application pool 'AppPool11' is being automatically disabled due to a series of failures in the process(es) serving that application pool."

"A process serving application pool 'AppPool11' suffered a fatal communication error with the World Wide Web Publishing Service. The process id was '3424'. The data field contains the error number. "

Just for the record, the server was a x64 windows server running among other things .net 1.1 web application.

To cut the story short, we were able to isolate the code that was throwing this error (a third party opensource dll called pdfsharp). The dll would work great as long it was saving/creating pdf, but as soon you tried the .Open command, either passing "stream" or "string to path" when called, the title blank application popup error was thrown only once on the system event viewer (until a reset on the worker was thrown by reseting the iis or killing the process) TIP: You can check the threads on a process with the sysinternals process explorer. In 32bit everything works fine (of course).

Interesting was that the request wouldn't catch that an error occured, meaning the thread would get stuck in the w3wp.exe worker. After a few hung threads (10?) the application pool would stop responding to requests and had to be terminated by the iis as shown.

Thank god we are working with 8 years old technologies, or our life would be pretty boring... not!