Why does this error occur?
According to Chrome Enterprise release notes:
CRX2 deprecation
Starting with Chrome 70, all non-force-installed extensions must be packaged in the CRX3 format. Extensions signed and hosted in the Chrome Web Store have been automatically converted.
Starting with Chrome 75, this restriction will also apply to force-installed extensions. Privately hosted extensions that were packaged using a custom script or a version of Chrome prior to Chrome 64.0.3242.0 must be repackaged.
If your organization is force-installing privately hosted extensions packaged in CRX2 format and you do not repackage them, they will stop updating in Chrome 75. New installations of the extension will fail.
However, it seems like in Chrome 73 Google forced this change for new installations already and introduced a new policy option for updates only.
But wait, it says all of them were converted automatically, so...
Why does it occur in your case?
According to resolution of this (not-a-)bug:
Beginning in M73, Chrome only allows installation of extensions signed with a CRX3 package. However, the webstore contains copies of both CRX2 and CRX3 versions (in order to accommodate older Chrome versions that can't handle CRX3), and by default, will serve CRX2.
The [...] package is using the old endpoint which will retrieve CRX2s, and so newer versions of Chrome will disallow the installation. [...] downloading libraries need to update their code to request CRX3s, and then this would work in all modern Chrome versions (CRX3s have been supported for several versions now).
So the problem is you were using an outdated URL to download extensions. See this answer for current URL format.
