My browser / operating system: Linux, Chrome 76.0.3809.132, No Flash version detected
I'm trying to answer a comment but when I open the message i get logged out

This has been a recurring issue for a while. Welcome to our community of sufferers!

Here are some possible solutions:

Ian-Stewart wrote:

If you look in the “Possible Solutions” section on this wiki article, there are two solutions to this problem. Hope I helped! :D

Scratch Wiki wrote:

Sometimes, if a user comments on one's profile later, the comment that previously would not show may appear. Then, the user can view, reply, delete, or report the comment. This works with most users, but is only temporary. This will need to be done again when another comment will not show.

Clearing the browser's cache may help fix this problem. If one uses Google Chrome, they can go to “Clear Browser Data” in Settings, click “Advanced,” then select the following:
Browsing History
Download History
Cookies and other site data
Cached images and files
To clear the entire cache, one can select “All time.” Then, they can click “Clear Data.”
Note Warning: Clearing one's cache may sign the user out of some websites, requiring the person to login again. Also, it may take longer to open some websites because images and other graphics will need to be reloaded.

Here's my giant post on the topic.

Hope this helped!