Before I started upgrading that WordPress plugin I did not actually notice, that I exceeded my disk quota. Well, sh*t happens.
Once I was able to think clearly again, I wondered which files I could easily get rid of. WordPress backups. I saved my (recent) WordPress backups over to my NAS immediately (I do have a Rakefile entry for that), and I removed the WordPress backups on my WordPress host. Now there was enough space to “breathe” again sufficiently.
$ cd .../wp/wp-content/plugins
$ mkdir tmp
$ mv THE_CORRUPT_PLUGIN tmp/
# from here on your "HTTP ERROR 500" should be gone already
# and your plugin appears as inactive on the plugins dashboard.
$ cd tmp/
$ wget FRESH_VERSION_OF_THE_CORRUPT_PLUGIN.tar.gz
$ tar xtf FRESH_VERSION_OF_THE_CORRUPT_PLUGIN.tar.gz
$ cd ..
$ mv tmp/FRESH_VERSION_OF_THE_CORRUPT_PLUGIN .
# re-activate your plugin on the plugins dashboard!
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