Team Foundation Server "cannot be migrated because the solution cannot be checked out" error

I solved this TFS problem:

Solution file 'whatever' cannot be migrated because the solution cannot be checked out from source code control. To migrate the solution, make sure the solution file can be checked out and re-open it.

By just unchecking the checkbox for read-only inspite of what these two links told me:

1. http://stackoverflow.com/questions/2762930/vs2010-always-thinks-project-is-out-of-date-but-nothing-has-changed
2. http://social.msdn.microsoft.com/Forums/en-US/tfsversioncontrol/thread/88d652fa-1fef-4e3c-8dcb-a2079a96f031 (suggested I uncheck "Allow editing of read-only files; warn when attempt to save" at Tools > Options... > Enviroment > Documents)