When you're downloading an I/O unit configuration file (OTG file), if PAC Manager appears to hang, it may be trying to process a corrupted configuration file (<filename>.otg file). This issue appears to be rare.
One of the indications of possible corruption is an unusually large file. Most I/O unit configuration files are very small, around 20 kB. If a file is larger than 1 MB, it is possible it is corrupted.
Make a backup copy of the I/O unit configuration file and then open the file with a text editor. Delete all instances of placeholder lines and then save the file. Placeholder lines look something like these:
Opto 22 is investigating this issue.