I needed to upload rather big file(s) to the server, so I added
<httpRuntime maxRequestLength="51200" />
to Web.Config file. After that I managed to upload needed file. But the problem is that after I added <httpRuntime maxRequestLength="51200" /> all other Submits began to result in sending empty http requests to the servers. At least it looks like that (Opera dragonfly showed empty http request, but debugger didn’t react to the breakpoint placed inside the controller, which must’ve happened; and no errors were displayed by opera).
I’ve got two forms on the same page, so this is why it didn’t work. After I removed one, everything started to work.