Update: We did clean-install Win7 and After Effects CC 2017 (AE only!) and kept the PC offline as long as possible and only went online to quickly activate AE. We then tried to open one of the AE projects, that I wasn't able to open previously. It did open! (But logically was missing dynamic links and Dolby codec etc.) We then saved to project under another name (let's just call it "project-working.aep" for now). After that we installed Adobe Premiere Pro and Media Encoder. And what do you know? The "old" problems are back and I cannot open any of the "old" projects. However, weirdly I can open the project I saved as "project-working.aep". However, the Premiere Pro project, that gets its dynamic links from that AE project, does not open (even after renaming the AE project).
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