@amalthia. Sometimes HTML has bugs in it that can cause parsing to hang. Can you keep an eye on the memory/cpu usage of your computer when you are importing the html to see if it is indeed one of these problems.
Another was to get more detailed debug output is to run the import from the command line as
calibredb.exe add "C:\path to the\html\file.html"
Or you could just opena ticket and attach the html file in question
