It's probably the index files.
MLVFS creates indexes of all your MLV files (if they don't already exist). These files are named the same as your MLV file, but end in .IDX. This indexing has to happen before MLVFS can give any information about a particular MLV file (this is because metadata and/or frames can appear anywhere in an MLV file, and the MLV format does not require that a frame count be in the first header).
If you put 500GB of files all in the same directory and then mount that directory with MLVFS, when the webgui comes up it wants to display information about all those files (same thing if you browse to the folder). So it tries to index all of them. For 500GB this will take a while. My guess is your issue has something to do with this (maybe you just didn't wait long enough, or there was some kind of timeout that cause MLVFS to crash).
Watch the folder where your original MLV files are located when you launch MLVFS. You should see these files starting to get created (if they are not already existing). Also keep an eye on disk, CPU, and memory usage of MLVFS. Then let me know what is going on.
The issue also could be related to corrupt index files. I would suggest you delete all the .IDX files before trying this. Then watch what happens as MLVFS creates them back.
One solution is to split up your files into separate subdirectories, that way MLVFS doesn't try to index all of them immediately all at once (it will only try to index files in a particular sub-directory if you browse to it).
Once all the index files are created, you won't have to wait anymore. My guess is this is why @silenttapes eventually got it to work. Each time he tried, MLVFS created a couple more index files, and eventually all of them got created.