Error when debugging: "Could not load web.config file. The given key was not present in the dictionary."

Could not load the Web.config configuration file. Check the file for any malformed XML elements, and try again. The following error occurred: The given key was not present in the dictionary.”

If your configuration file is well-formed, possibly this error is happening due to your public URL set on Central Administration, on Alternate Access Mappings. The Site Url property of your project in Visual Studio needs to match the public URL on AAM. Check it on Central Administration > Configure alternate access mappings > Edit public URLs.

When updating data inside a collection: "Collection was modified; enumeration operation may not execute".

When trying to update some data in a collection, you will find this error:

 foreach (SPListItem item in list.Items){
item["column"] = "New title";
item.Update();
}

You can´t update read-only collections. The solution: keep the ID of the ListItem you want to modify in some kind of object (like a generic list, for example), and modify this item in one-way, out of any collections, getting it by its ID.

SPListItem item = list.GetItemById(idItem);
item["column"] = "New title";
item.Update();

When running psconfig (Products Configuration Wizard), and providing database server name, the following error appears: SQL Server at [server name] has an unsupported version".

See the minimum versions of SQL Server required to the SharePoint 2010 installation on same farm:

•SQL Server 2008 Express Edition Service Pack 1, version number 10.0.2531;
•SQL Server 2005 Service Pack 3 cumulative update package 3, version number 9.00.4220.00;
•SQL Server 2008 Service Pack 1 cumulative update package 2, version number 10.00.2714.00.

It´s very important to remember that SQL Server SP1 is necessary on SQL Express. If SQL version is paid (Professional or higher), it´s needed the SP1 Cumulative Update 2 too.

While using Windows Server: "Your client does not support opening this list with Windows Explorer".

On Server Manager, enable the "Desktop Experience" feature. Then, enable the "WebClient" Windows service, on services.msc.

While adding content DB: "This content database has a schema version which is not supported in this farm."

Install all Cumulative Updates on SharePoint servers, until its version be the same version as the old farm.

Error opening Excel file on SharePoint: "The workbook is currently open by 256 users. A maximum of 256 users can have a shared workbook open at the same time."

The file you are trying to open is, somehow, corrupted - unless it´s being opened by 256 users, obviously. In case of corruption, you can try to save a copy of the file on SharePoint, using the Excel. Ignore the "read-only" warning and "save as" a new file. Delete the old one. Solved!

When adding a content database: "The specified SPContentDatabase Name=(name) Parent=SPDatabaseServiceInstance has been upgraded to a newer version of SharePoint".

SharePoint´s version on the machine where you try to add the content DB is older than in the other machine. Install the Service Packs and the Cumulative Updates needed to reach the same version.