Why does using "a section registered as allowDefinition='MachineToApplication' beyond application level" become an error?

Question

I had this ASP.NET project/site building and running earlier; with no code changes, now it doesn't. I've restarted Visual Studio, and even restarted the computer, but no difference. Now if I select either Build Project or Build Website, I get:

Error   1   It is an error to use a section registered as allowDefinition='MachineToApplication' beyond application 
level.  This error can be caused by a virtual directory not being configured as an application in IIS.  C:
\MembersOrderEntryRedux\MembersOrderEntry\MembersOrderEntry\Mobile\web.config   58

...which goes to this line:

<authentication mode="Windows"/>

...in \Mobile\web.config

I looked at this, but it didn't help - some of the suggested fixes and workarounds don't apply to my project/website (ASP.NET 3.5) for some reason - I have no OBJ folder, my web.config files don't expose the properties discussed there, etc.

Does anybody know what would cause a project that was running to no longer build and throw and error this way? The only things I changed (after this problem turned up) was to check the "Build" checkbox in Configuration Manager (even though the solution only has one project, it wasn't selected for some reason).

NOTE: The project's "Mobile" folder has a "bin" subfolder, and it contains a gazillion DLLs with names like App_Web_9m76tru72.dll

What the heck are these, and do they belong there?

Answer 1

In my particular case, I had to exclude stuff that was giving the compiler conniption fits

Excluded all these files beneath the "Mobile" folder (which has/had a second web.config file, also has a "bin" folder of beaucoup macabrely-named .dlls, such as "App_Web_2zzrae4.dll" and such.

Default.aspx
Default.aspx.vb
Default_Old.apsx
web.config

I also did the following, which may have contributed to the ultimate rehabilitation of the project/site:

Deleted the website.publishproj file
Deleted the *.suo file[s]
Removed TFS entries from .sln file

UPDATE

Based on something I found here, simply changing the name of the file that was throwing this (from web.config to mobileweb.config) allowed it to work.