[bobalazs]

Only DVI monitor connected to vga card, igpu disabled in bios.
Kodi worked for months until one day decided it wants to display on igpu vga output.
That resulted in ERROR: Unable to create GUI. exiting
if i wanted to, one solution would be to connect a vga cable to the motherboards igpu vga connector. Then i'd enable the monitor on it, in windows, that kodi would start on that screen.
However, there is now a different solution.
I backed up the windows' username/appdata/kodi folder from when it was working properly and compared it to the username/appdata/kodi folder from the "Unable to create GUI. exiting"  bugged out version.

userdata folder has guisettings.xml file that i opened in notepad++; specifically the videoscreen part that is interesting.
this is one that runs in windowed mode and is working

 <videoscreen>
        <blankdisplays default="true">false</blankdisplays>
        <cms3dlut default="true"></cms3dlut>
        <cmsenabled default="true">false</cmsenabled>
        <cmsgamma default="true">220</cmsgamma>
        <cmsgammamode default="true">0</cmsgammamode>
        <cmslutsize default="true">6</cmslutsize>
        <cmsmode default="true">0</cmsmode>
        <cmsprimaries default="true">0</cmsprimaries>
        <cmswhitepoint default="true">0</cmswhitepoint>
        <delayrefreshchange default="true">0</delayrefreshchange>
        <displayprofile default="true"></displayprofile>
        <dither default="true">false</dither>
        <ditherdepth default="true">8</ditherdepth>
        <fakefullscreen default="true">true</fakefullscreen>
        <limitedrange default="true">false</limitedrange>
        <monitor default="true">Default</monitor>
        <noofbuffers default="true">3</noofbuffers>
        <preferedstereoscopicmode default="true">100</preferedstereoscopicmode>
        <resolution>15</resolution>
        <screen>-1</screen>
        <screenmode>WINDOW</screenmode> 
        <stereoscopicmode default="true">0</stereoscopicmode>


Compare it with fake-full-screen- Unable to create GUI. exiting solution version of the config file  

<videoscreen>
        <blankdisplays default="true">false</blankdisplays>
        <cms3dlut default="true"></cms3dlut>
        <cmsenabled default="true">false</cmsenabled>
        <cmsgamma default="true">220</cmsgamma>
        <cmsgammamode default="true">0</cmsgammamode>
        <cmslutsize default="true">6</cmslutsize>
        <cmsmode default="true">0</cmsmode>
        <cmsprimaries default="true">0</cmsprimaries>
        <cmswhitepoint default="true">0</cmswhitepoint>
        <delayrefreshchange default="true">0</delayrefreshchange>
        <displayprofile default="true"></displayprofile>
        <dither default="true">false</dither>
        <ditherdepth default="true">8</ditherdepth>
        <fakefullscreen default="true">true</fakefullscreen>
        <limitedrange default="true">false</limitedrange>
        <monitor default="true">Default</monitor>
        <noofbuffers default="true">3</noofbuffers>
        <preferedstereoscopicmode default="true">100</preferedstereoscopicmode>
        <resolution default="true">16</resolution>
        <screen default="true">0</screen>
        <screenmode default="true">DESKTOP</screenmode>
        <stereoscopicmode default="true">0</stereoscopicmode>
    </videoscreen>


The specific lines that i deleted now are 

<resolution default="true">16</resolution>
        <screen default="true">0</screen>
        <screenmode default="true">DESKTOP</screenmode>

and replaced them with 

  <resolution>15</resolution>
        <screen>-1</screen>
        <screenmode>WINDOW</screenmode> 
        <stereoscopicmode default="true">0</stereoscopicmode>

Once i saved the file guisettings.xml this way, i could start kodi in Windowed mode, no more error.
Give it a shot.