At it worked before something must be damaged. Try
1. Reinstalling to have binaries in correct state.
2. Delete registry for host (regedit.exe -> /HKCU/Software/RepetierHost or rename it for just testing)
3. In admin console run "sfc /scannow" to check windows/.NET binaries are not corrupted. Takes a while and gives a long report on disk so read better the critical output on console.