I installed eclipse in / opt as root. I had this exact problem.
This seems to have something to do with permissions (yes, great error message, I know).
I just installed plugins during eclipse startup as root:
user@computer ~$ sudo eclipse &
The problem is solved!
EDIT: Instead, when you install it, you can change the ownership of the eclipse files so that you do not have to run the superuser every time you want to install the addon.
user@computer /opt $ sudo chown -R user eclipse/
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