When I tried to install a Google Chrome extension in the latest dev version of the web browser I was notified that an error had occured. The message below that stated that the download was not a CRX. Reloading did not fix the issue, neither did a restart of the browser, and next thing I tried was to install another extension from the official web store to see if it did install in Chrome. I did receive the same error message which puzzled me. Somehow Chrome was not download an extension file, which do have the crx file extension, but something else.
I fired up Chrome Canary, another version of Chrome that I use rarely to see if I was able to install extension in the browser. Turned out, I received the same error messages when trying to install extensions in the browser too.
The download directory was listing the crx file downloads, but attempts to install the extensions in Chrome revealed that they appeared to be corrupt. For one, they had a slightly different file size which is obviously a bad sign if it is the same file download.
I tried a lot of things afterwards, clearing the cache, trying to find the download url of the crx file manually and all of that stuff. Last thing I tried, and that is one of the things that still puzzles me, is to sign in to a Google account. Guess what, after signing in the installation went through without issues.
I tried it again in the second version of Chrome that I was not signed in and it failed again. Signing out in the other browser and signing in again on the other hand let me download and install the extension just fine a second time.
I then signed in on the second Chrome browser and here too, the installation went through without issues. I can't say what is causing this but if you run into the error, I suggest you check if you are signed in and if you are not, sign in to see if it fixes the issue on your end.
Update: As some users have pointed out, going into Incognito Mode with Ctrl-Shift-N, loading the page of the extension in that mode and clicking on install then should also fix the issue.
Update 2: Note that signing out and signing in again may also work in case you are already signed in to your Google account. As some users have pointed out, there may be other solutions like deleting a user account or moving the download directory from a network share to a local folder.