what is the purpose of the rptproj.user file

rdl files are critical and should be recorded in source control

.rdl.data is useful only in preview (saving trips) and therefore should not be recorded in source control

.rptproj file has useful datasource, path and deployment info and should be recorded in source control

.sln contains useful solution container info and should be recorded in source control

.suo contains user specific setting/preference info and should not be recorded in source control

.rds contains important data source info and should be recorded in source control

rptproj.user is a user specific data file which by design stores data sources and the like so a developer cannot steal another developer's credentials. Visual Studio checks if the user file (where appropiate definitions for a single user is stored) stores some information related to you. So you either need to have NTFS permissions to read the information or delete / rename the file.