Make a Backup of those two files and replace them with the original files ...
if the delete process works, there might be a compatibility problem with a applied hack. Compare the two files to see the diference between the two files. Re-appy each change you made to the old version (one at a time and save) to the original version to locate the source error.
Test each sequence (creating test threads to delete) untill you find the root of your problem ...
Hope this helps!