Hi @r.lennart.johansson
Welcome to the LastPass Community.
The instructions for adding secure notes varies slightly depending upon the platform you're using LastPass on. In your case, I would first suggest rebooting the affected device, log back into LastPass, then add the secure note again.
If that should fail, please test an alternate network connection such as LAN Vs. Mobile data.
If you continue to receive the error, please Contact Us directly to create a support case.