We have been discussing this topic for a while in the German Onleihe User Forum, but nobody seems to be able to resolve it.
As it looks right now, everybody with Mac OS X 10.11.2 (or the latest release of Yoseimte aswell) get the same kind of problem when downloading (fulfilling) epub books from the "Onleihe" (which is the provider for eBooks for most of German libraries): Books are downloaded succecssfully (as it seems), the progress bar advances until the end, but then the application just doesn't do anything more. The only option is to forcefully close ADE. This happens both with ADE 2.0 and 4.5, but apparently not with 1.7.2 (which is outdated and cannot be downloaded officially from Adobe anymore).
We dug into what could be the reason for this failing, and ended up with the Mac OS X security update from mid December; after installing this, is seems that the problems started to occur. We are talking about this one:
Informationen zum Sicherheitsinhalt von OS X El Capitan 10.11.2 und zum Sicherheitsupdate 2015-008 - Apple Support
Even more favoring this as a reason for failing, a Lion system I have here (Mac OS X 10.7.5) still works, with ADE 2.0.67275.
Checking the logs using "console" in Mac OS X, I see the following:
22.12.15 20:10:47,005 Adobe Digital Editions fulfillmentCallback, newState:422.12.15 20:10:47,006 Adobe Digital Editions fulfillmentCallback, newState:4
22.12.15 20:10:47,007 Adobe Digital Editions fulfillmentCallback, newState:4
22.12.15 20:10:47,008 Adobe Digital Editions fulfillmentCallback, newState:4
22.12.15 20:10:47,008 Adobe Digital Editions fulfillmentCallback, newState:4
22.12.15 20:10:47,009 Adobe Digital Editions fulfillmentCallback, newState:4
22.12.15 20:10:47,024 Adobe Digital Editions fulfillmentCallback, newState:4
22.12.15 20:10:47,024 Adobe Digital Editions connectionDidFinishLoading
22.12.15 20:10:47,085 Adobe Digital Editions fulfillmentCallback, newState:9
---------------- this is the last line I would see on 10.11.2, the rest comes from 10.7.5
22.12.15 20:10:47,123 Adobe Digital Editions unencode url string is [url]http://acs4.onleihe.de/fulfillment/FulfillmentNotification[/url]
22.12.15 20:10:47,123 Adobe Digital Editions encode url string is [url]http://acs4.onleihe.de/fulfillment/FulfillmentNotification[/url]
22.12.15 20:10:47,124 Adobe Digital Editions FulfillmentProgressController::onCallback, newState:9
22.12.15 20:10:47,160 Adobe Digital Editions connectionDidFinishLoading
22.12.15 20:10:47,179 Adobe Digital Editions unencode url string is [url]https://ws.onleihe.de/acs4return[/url]
22.12.15 20:10:47,179 Adobe Digital Editions encode url string is [url]https://ws.onleihe.de/acs4return[/url]
22.12.15 20:10:47,602 Adobe Digital Editions connectionDidFinishLoading
22.12.15 20:10:47,606 Adobe Digital Editions fulfillmentCallback, newState:6
Apparently something fails, hangs, is fenced off by Mac OS X or whatever... Note that there is no security log message from Mac OS X in the logs (I could see at least).
I hope this is resolved soonish; it's really tedious to have to either use an outdated version of ADE, or dust off a real old and slow computer just to be able to fulfill library books.
Thanks in advance for your support!
Best regards, Martin