virtualbox.org

I went through the whole thread, but couldn't find a solution. What I understood from there is that I need to have a reproducible test case. I finally could find a combination that always produces two or three characters instead of one, it's Alt+Tab for me (or Option+Tab in MacOS terms).

The logs are attached.
VBox version is 5.1.26 r117224.
Guest additions installed is the same as VBox.
The guest is a Slackware Linux with kernel 4.4.

You don't even describe the problem in full detail. Could you do that? Symptoms. Don't have me go reading other threads when I don't particularly need to. Just your case in detail would suffice.

Same about the reproducible case.

Sorry, if I was not clear enough. The problem is pretty simple and it's in the subject: typing fast produces several characters for a single key stroke. When I press Alt+Tab I get a windows switched and a few more Tabs printed in the second window. If with other keys it's not always reproducible, with Alt+Tab it has the same effect in 80% of the cases.

I haven't seen this. What I have seen recently is a ticket (https://www.virtualbox.org/ticket/17051) that's discussing keyboard issues. Not related to yours, but it shows the route that you have to take in order to debug the issue. See if anything in there can help you.