this message appears when I'm trying to login.

Are you using Windows XP? Please make sure you have installed the latest Service Pack and Internet Explorer.

This error usually occurs on old Windows XP systems that do not support recent SSL (encrypted network connections).

I have latest servicepack.
Problem appears two days ago

infinitive1
I have latest servicepack.

make a check with linux on live usb (this usually does not interfere with your os installed on your hard disk) too see if there is a hardware problem (like missing drivers)

infinitive1
I have latest servicepack. Problem appears two days ago

There have been no SSL encryption changes on the jammr.net server side. The issue is probably related to your Windows XP installation.

Note that in addition to the latest Service Pack you also need the latest Internet Explorer.

If you have access to a newer version of Windows then that will solve the problem too.

I have no changes on my side.
The jammr service was OK on monday, but now it always reports ‘error 6’…

Are you able to access https://jammr.net/ from Internet Explorer on that machine?

(It's important to use Internet Explorer and not another browser like Firefox or Chrome because they use different SSL implementations and may not see the same error as jammr and Internet Explorer.)