It seems that the login problems mainly happen when switching between Mir and standard X, proceeding as described on https://wiki.ubuntu.com/Mir/Installing, section "Removing unity-system-compositor"/"Temporarily", especially when deactivating Mir, returning to standard X. Additional reboots, or selecting an alternative session instead of standard Ubuntu/Unity, like XBMC, helps to get logged in again.
It seems that the login problems mainly happen when switching between Mir and standard X, proceeding as described on https:/ /wiki.ubuntu. com/Mir/ Installing, section "Removing unity-system- compositor" /"Temporarily" , especially when deactivating Mir, returning to standard X. Additional reboots, or selecting an alternative session instead of standard Ubuntu/Unity, like XBMC, helps to get logged in again.