Azure devops server hangs on send work items in email

we have recent upgraded from TFS to Azure Devops server 2020.01.

Now we encounter an issue that when we want to mail work items it stays on 'please wait'. Occasionally it works, but largely not. Already tried with different browsers.

Within chrome we do see this error message:

Content?bundle=vss-bundle-async-vlBqqqCimG3tKYci5_YvSYAaTiC55_my7aZvW0q0W6eo=:17 Uncaught TypeError: Cannot read property 'guid' of undefined at t.sendMessage (Content?bundle=vss-bundle-async-vlBqqqCimG3tKYci5_YvSYAaTiC55_my7aZvW0q0W6eo=:17) at Content?bundle=vss-bundle-async-vlBqqqCimG3tKYci5_YvSYAaTiC55_my7aZvW0q0W6eo=:15 at n. (Content?bundle=vss-bundle-common-vG0KUJjg3--UYqBo9qQKb4H_-6CYjE75N8kARpETg7Vg=:179) at n._func (Content?bundle=vss-bundle-common-vG0KUJjg3--UYqBo9qQKb4H_-6CYjE75N8kARpETg7Vg=:77) at n._invoke (Content?bundle=vss-bundle-common-vG0KUJjg3--UYqBo9qQKb4H_-6CYjE75N8kARpETg7Vg=:77) at Content?bundle=vss-bundle-common-vG0KUJjg3--UYqBo9qQKb4H_-6CYjE75N8kARpETg7Vg=:77