When I start a project in monoDevelop, it is showing this error. But there is no space or any illegal character: see screenshot
This is a bug in MonoDevelop 5.10 which was fixed recently.
MonoDevelop 5.10 does not allow the '#' and '%' characters to be used in the project, solution or location text boxes in the New project dialog.
Confusingly the error message mentions the project name when the actual problem is the location.