Mailbox Text only react when Shift-key is pressed

I'm fairly new to Mailbox as I'm moving away from Google. Until now I'm pretty excited.

However, Mailbox Text is useless to me, and that's a pretty serious flaw. The problem is that if I open a new or an existing text document, I can type as much as I want on the keyboard, but nothing is registered in the document ... unless I press the SHIFT-key. Then I can get the uppercase letters. If I want lowercase letters I can activate CAPS LOCK and press the SHIFT-key while I type.

Of course that doesn't work in the long run, so this is kind of a deal breaker for me.
I'm also puzzled why the default font color is a very faint light grey that is very hard to read. I don't know whether that's coupled to the other problem.

With this type of problem you would assume that it is either a HW issue or some kind of configuration issue. However, I don't have any problem in all other Mailbox applications like e.g. Spreadsheet.

I'm running Mailbox via the Vivaldi browser on Debian Linux and I don't have this issue in any other application.