UMAT refcount error with setUseOpenCL(false)

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Currently I'm rewritting some code from Mat to UMat that will hopefully run both with or without OpenCL depending on the OpenCL flag. Now I'm getting a refcount error I don't quite understand. It only shows up if setUseOpenCL is set to false.

(Since the images I work with are around 196Megapixel-RGB-images I do prefer avoiding deep copies as much as possible and I would like to understand why in the code below I'm not allowed to write into the orginal UMat.)

By the way: is there an easy way to simply convert any image (gray, bgr, bgra) to gray or even directly to CV_32FC1 or do I always have to manually check if the image is bgra or bgr or gray and then do a conversion?

main()
{
   ocl::setUseOpenCL(false); //myFoo() crashes only if this is set to false.
  myFoo(myimg, outImg);
}
 
//in this function the line convertTo() crashes if the input UMat is the same as the output UMat and setUseOpenCL is set to false
void myFoo(InputArray img_ext, OutputArray img_Out)
{
    UMat gray; //if I use Mat instead of UMat it doesnt crash.
    img_ext.copyTo(gray); //I also tried using getUMat(), but in both cases convertTo() below crashes.
 
cvtColor(gray, gray, COLOR_BGR2GRAY); //doesn't crash
 
UMat gray2;
    gray.convertTo(gray2, CV_32FC1); //this doesnt crash
 
gray.convertTo(gray, CV_32FC1); //crashes with error message: OpenCV Error: Assertion failed (u->refcount == 0) in cv::StdMatAllocator::deallocate