Sharepoint - Server Error in '/' Application. Runtime Error

In short, there's another web.config file in the LAYOUTS folder in the 14 hive that you need to change, it's set out fairly well here: http://web.archive.org/web/20130909002813/http://www.khamis.net/blog/Lists/Posts/Post.aspx?ID=12

To solve this error you have to change settings in both the web.config file that one is in the IIS folder and the other web.config file which is in the LAYOUTS folder 15 hive.

Simply Change:

<customErrors mode="Off"/>

You need to debug properly without changing the settings!

firstly, what web.config are you modifying? where is it located and what version of sharepoint are you using?

if its a webpart that your using than the web.config you need to edit will be the web applications web.config.

goto IIS manager, web sites -> select the website in question -> right click and select open.

if its not a webpart but an actual application (mapped application in iis), it would have its own web.config:

goto IIS manager, web sites -> select the website in question -> click the plus sign next to it(expand) you should see some files and folders.... what your looking for is web applications, find the one your using -> right click it and select open.... you should see .cs and aspx files and folders... you should also have a web.config there. If not put one in.

if its a stand alone aspx (on its own within 12/14/15 hive) than it will be most likely under layouts folder, that will use the first one i stated above.

you need to put in the web.config:

        <customErrors mode="Off"/>

(which you have done)

if its an application, have you debugged the code? by attaching to process and putting a break point within the code you can see where abouts its breaking!

have you looked at event viewer on the server to see the errors that are being procduced? uls viwer?

if it is an application make sure that the project within iis manager is set as an application with the appropriate app pool