So I opened the .env-file and it says:
PATH=C:\OSGEO4~1\apps\qgis\bin;C:\OSGEO4~1\apps\Python36;C:\OSGEO4~1\apps\Python36\Scripts;C:\OSGEO4~1\apps\qt5\bin;C:\OSGEO4~1\apps\Python27\Scripts;C:\OSGEO4~1\bin;C:\WINDOWS\system32;C:\WINDOWS;C:\WINDOWS\system32\WBem GDAL_DATA=C:\OSGEO4~1\share\gdal GDAL_DRIVER_PATH=C:\OSGEO4~1\bin\gdalplugins GDAL_FILENAME_IS_UTF8=YES JPEGMEM=1000000 OSGEO4W_ROOT=C:\OSGEO4~1 PROJ_LIB=C:\OSGEO4~1\share\proj PYTHONHOME=C:\OSGEO4~1\apps\Python36 QGIS_PREFIX_PATH=C:/OSGEO4~1/apps/qgis QT_PLUGIN_PATH=C:\OSGEO4~1\apps\qgis\qtplugins;C:\OSGEO4~1\apps\qt5\plugins QT_RASTER_CLIP_LIMIT=4096 VSI_CACHE=TRUE VSI_CACHE_SIZE=1000000
What I have done so far:
1. uninstalling the program and reinstalling several times via Standalone-installer aswell as the OSGEO4W-installation - also tried the three different server and with Administrator-rights (as described in Problem starting QGIS 2.14, Prozedureinsprungpunkt is not found)
2. uninstalled and cleaned the registry-data as described in How to completely uninstall and remove QGIS 2.8.1?
3. I searched for a duplicate of the Qt5Sql.dll that appeared another time in my Teamspeak-folder > so I uninstalled Teamspeak
4. Searched for leftovers from old versions and deleted all
5. uninstalled the QGIS-version via the OSGEO4W-installer and deleted the OSGEO4W-folder
6. installed the 3.0 (32-bit version) and later the 2.18.18-LTR version (no problems appeared there)
If anyone has another idea how to solve the problem, I am happy for your help!