WindowServer stuck during booting - only safe mode works

Question

Since today during working it seems the WindowServer stopped functioning.
I can only start the Mac with Shift-Down = Safe Boot. Then it works with all connected screens, but is obviosley unusable, because nothing is working without the extensions.
So I tried to install a new system onto another drive.
Same Problem - Also I can start the OSX installer from the 3rd drive with only pushing the shift key.
It´s the same thing again and again.
Booting - gray screen. In this state I con logon via ssh and can see the process WindowServer stuck. I cannot kill it.
shutdown -r now won´t work.
What is the problem ? - please help !
windowserver.log says:
Mar 03 00:24:05 [63] CGXRestartSessionWorkspace: session workspace exited for session 257 ( on console )
Mar 03 00:24:05 [63] loginwindow connection closed; closing server.
Mar 03 00:24:44 [71] Server is starting up
Holding the shift key it says:
Mar 02 15:08:50 [63] CGXMappedDisplayStart: Display0 : boot screen
Mar 02 15:08:50 [63] CGXMappedDisplayStart: Display0 : no display alias property
Mar 02 15:08:50 [63] CGXMappedDisplayStart: Display1 : no display alias property
Mar 02 15:08:50 [63] CGXMappedDisplayStart: Display2 : no display alias property
Mar 02 15:08:50 [63] CGXMappedDisplayStart: Display3 : no display alias property
Mar 02 15:08:50 [63] AGCAttach: Couldn't find any matches
Mar 02 15:08:50 [63] CGXPerformInitialDisplayConfiguration
Mar 02 15:08:50 [63] Display 0x1cab7674: MappedDisplay Unit 0; Vendor 0x472 Model 0xad95 S/N 1916801846; online enabled (0,0)[1440 x 900], Rotation 0, base addr 0x106800000
Mar 02 15:08:50 [63] Display 0x3f003f: MappedDisplay Unit 3; Vendor 0xffffffff Model 0xffffffff S/N -1; offline enabled (2464,0)[1 x 1], Rotation 0, base addr 0x116800000
Mar 02 15:08:50 [63] Display 0x3f003e: MappedDisplay Unit 2; Vendor 0xffffffff Model 0xffffffff S/N -1; offline enabled (2465,0)[1 x 1], Rotation 0, base addr 0x116800000
Mar 02 15:08:50 [63] Display 0x3f003d: MappedDisplay Unit 1; Vendor 0xffffffff Model 0xffffffff S/N -1; offline enabled (2466,0)[1 x 1], Rotation 0, base addr 0x106800000
Mar 02 15:08:51 [63] Display 0x1cab7674: MappedDisplay Unit 0; ColorProfile "Default(sRGB IEC61966-2.1)"
Mar 02 15:08:51 [63] Display 0x1cab7674: MappedDisplay Unit 0; ColorProfile "X192W"

Answer 1

Having a loading bar at startup in OS X Yosemite doesn't mean that you are booting into Safeboot. (As its the new design of the OS) .
Follow the steps in Resolve startup issues and perform disk maintenance with Disk Utility and fsck and  see how it goes.