Index not found. (Exception from HRESULT: 0x80131124)

I have created a VSTO Plugin for Outlook 2010. Now tried to convert this VSTO Plugin from VS 2010 to VS 2016 and to Outlook 2016. After the default conversion of visual studio I run my build and I get the error: Index not found. (Exception from HRESULT: 0x80131124)

I figured out if i have more the 3 windows forms my build throws this error. If I inlcude only 3 forms the build runs fine. And it is not a problem of a specific form because I can use randomly 3 of my forms and the build works fine. After adding only one other build is broken.