Safari - installation of third-party-extensions not possible

Hello community,

since the update to Mac OS X El Capitan 10.11.3 I'm unable to install Safari extensions from third parties (like Agilebits 1Password or ublock).

I can double click the extension and the dialog wether I trust trust the source is displayed (it's in german).

But regardless of what choice I make the extension ist not installed.

My system is

• iMac (Retina 5K, 27-inch, Late 2014) - 4 GHz Intel Core i7, 32 GB 1600 MHz DDR3, 3TB Fusion Drive, AMD Radeon R9 M295X 4096 MB
• Safari 9.0.3 (since the combo update to 10.11.3)

I've tried to

• restart the iMac
• install the 1Password-4.5.2.safariextz file from different locations
• delete the extended attributes and ACLs from the downloaded 1Password-4.5.2.safariextz file
• uninstall all extensions and empty the folder ~/Library/Safari/Extensions
• install the extensions via Terminal (open ~/Downloads/1Password-4.5.2.safariextz or sudo open ~/Downloads/1Password-4.5.2.safariextz)
• copy the 1Password-4.5.2.safariextz directly in ~/Library/Safari/Extensions
• download the 1Password-4.5.2.safariextz again from a different location

Google also did not show any relevant problem solutions to my problem.

Currently I'm only able to install extension directly from the Apple Extensions Gallery.

The problem is that the 1Password extension from Apple Extensions Gallery is out of date and does not work properly with 1Password 6.0.1 (problems on amazon.com and other sites). Also is ublock not available from the Apple Extensions Gallery.

Has anybody else had similar problems and could solve them?

Best regards

Gerhard