You didn't provide the c2addon file you were using, [...]. Presumably this is a problem with the specific addon you are trying to install.
I've tried many addons, so it's not the addon's fault
Basicly, I have the same error with all addons I tried.
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Problem Description
Can't import c2addon
Steps to Reproduce Bug
Observed Result
Expected Result
Worked as expected (https://www.scirra.com/manual/158/third-party-addons) in the past, I do not remember what C2 revision I used the last time I tried to install a plugin, I upgrade C2 at every beta more or less
Operating System
Windows 10
Construct 2 Version ID
Construct 2 beta r247