The extension you are trying to install is digitally signed. The failure of installation can occur due to the following reasons.
1. Either the package has changed since its packaging.
2. Either the signature is invalid.
If you have acquired the extension from a third party, please verify that the contents of the package you are using are correct or that the package has not been altered with.
The problem seems to be different because the extension works correctly on all computers except two where there is this message.
So, we are trying to find differences between computers where extension works correctly and the two others but no differences ( same OS, same indesign build, same ExtensionManager build ... )
Our certificate is signed.