I had a major issue opening up an older project today. I went through multiple reinstalls, Adobe CC Cleaner tool, a round with Adobe support because my apps were suddenly not working even after installing (that's another long story) etc. without any remedy before coming to this resolution. A full 5 hours spent on this issue.
At first, I thought it might be Premiere linked comps so I copied the AE project over to my desktop and unmounted the original drive to force a disconnect between the project and any source files. I then opened the AE project without any issue so I re-saved under a new name and deleted all 'Dynamic Linked' Premiere comps that appeared in the project. Re-mounted the drive and launched the new version of the project but it wouldn't open (curiously, it used to get to 53% before hanging with the "1 Sec remaining' message but now it would hang before the progress window could finish drawing itself - I had just a white rectangle where the window was supposed to be). I force quit After Effects (only way to close it with the troublesome project), unmounted the source drive again and then opened the 'disconnected' project.
This time, I chose the comp that I actually need to make changes to and consolidated it (under dependancies>collect files). Then I closed down AE (for kicks and giggles), restarted it and opened the consolidated project with the source drive still unmounted. At this point, the consolidated project consists of just 1 piece of Red footage, another quicktime file in h.264 codec, 1 master composition, 1 nested composition for mocha tracking on the Red footage, an Illustrator doc imported as a composition and a handful of solids.
First, I remounted the source drive and tried to reload footage (right click>reload footage). This promptly caused AE to hang. Yay. Went through the force quitting/unmounting/reopen process again. This time, I tried just importing the clips in question and got sent down a rabbit hole as the very first clip I tried to bring in hung AE. After going around in circles and then trying to take that same clip into Media Encoder, I had an 'Ah-ha' moment. This clip was suffering from the issue that's been going around in various AE discussion circles since the last update (re: dropped support of 'legacy' codecs). Although it is supposedly under the supported codecs banner (again, this clip is in h264 codec), for whatever reason the Adobe products do not like it. How do I know? Well, trying to bring that same clip into AME in order to create a fresh test one caused it to crash immediately - I mean, as soon as I dropped the clip into the AME window, AME hard crashed. This was repeatable with this clip but couldn't be replicated with other random h264 clips I tried.
So, I took the clip in question and opened it in my still-somehow-laying-around QT7 application and exported as, well, another mp4 file. After re-opening the AE project, importing the new mp4 clip, replacing any instances of it in compositions (option drag from project window to selected/highlighted clip in comp to replace in place) and then reloading the rest of the footage, everything appears to be ok.