After upgrading the IIS, we've got the following error (when trying to connect to the web service).
Could not load file or assembly 'Microsoft.TeamFoundation.
WorkItemTracking.Client. DataStoreLoader' or one of its dependencies. An attempt was made to load a program with an incorrect format.
How to fix:
You need to enable the IIS application pool to run in 32bit.
IIS->Application pools-> right click on the relevant application pool->advanced settings-> change "Enable 32-bit Application" to "true"