I doubt it's reproducible, but here's what I did:
- open QC with an existing session (which basically contains one project (QC but with another build directory))
- had code model issues... [don't know if it matters, but got no highlighting, auto completion or else, lots of restarting clangbackend on cmdline and General Messages]
- closed Project with File -> Close Project "qt-creator"
- switch to Welcome screen
- click the top most project to re-open the closed one