I have got the same problem and found a solution: Change the script clearSafariCache.sh from

find /private/var/folders \! -name "*SafariTechnologyPreview*" -name "*com.apple.Safari*" -exec rm -r {} \;

to

find /private/var/folders \! -name "*SafariTechnologyPreview*" -name "*com.apple.Safari*" -exec rm -r {} \; 2>/dev/null

Because the original script throws "Permission denied" errors and therefore opens the file descriptor stderr, it seems that the parent process (com.apple.foundation.UserScriptService) can't close it as long as a file is open.