Rename your ~/.gimp-2.8 folder to maybe ~/.gimp-2.8.bak
Start Gimp and it will make a new default (profile) ~/.gimp-2.8 ie. same as a new install.
If you want single window mode, enable in the Gimp -> Windows menu.
If all is working, copy any brushes, plugins, scripts etc. to your new profile.
delete the backup profile.