I'm still trying to find out if it's a mod_security rule issue, but in the meanwhile I figured out a work-around:
Download the extra(s) you want from the MODx Extras website:
http://modx.com/extras/
From the Manager upload the transport zip(s) into the core/packages folder by right-clicking the packages folder and choosing Upload Files.
Then go to Package Management and find the green Download Extras button. Click on the little arrow and from the drop-down choose Search Locally for Packages.
The uploaded packages will show up and you can install them from there as usual.
Hope this helps!
[ed. note: redhen last edited this post 9 years, 8 months ago.]