Thanks a lot. EscapeHTML() fixed the issue. 

I don't know if it's same case as OP but I got "ERROR #5001: Create Directory failed: _err" as well when the method System.OBJ.ExportUDL() is called in parallel. Looks like there is a race condition. Create directory might fail is another process is already creating or has created that directory.

Thanks for the suggestion. I have tried to group CLS files to be loaded into clusters of 256 items, each cluster is then sent to a worker (instead of worker getting one CLS at a time). This increase chance of worker working exclusively on one package. In the end it's roughly same time. I don't wanna load them by package as packages are not balanced (some have 10 classes, some 500).