Those users who have access to 6.3.1 may download fixes I made over it to improve thread-safety and to fix other issues.
Please download from Source Forge https://sourceforge.net/projects/opencascade/ using 6.3.1 subfolder.
As usual, readme.txt contains details on made modifications.
The patch can be applied directly over 6.3.1 installation. There are source-only and source+binary (built with MSVS2003) packages. Choose what suites best for you.
Hope this helps.