After some minor change of a master page in SharePoint designer, I checked it in as a major version. SharePoint designer then opened the library "Master Page Gallery" ( /_catalogs/masterpage/Forms/my-sub.aspx ).
I can see the two changed master page files ( .html and .master ) in the "My submissions" list view. However, when trying to open the context menu to approve it, I got the error message "This item is no longer available".
I logged on as SharePoint farm administrator, so it should not be caused by permission issue.
Google leads me to this one, but it doesn't help.
In the end, I changed the versioning settings of this library, which fixed the problem.
Then I changed the settings back.
This is the first time I got this issue in the past 9 years. I guess it's caused by a minor bug. As alternative solution, it's good enough.
The environment is SharePoint 2016 with CU 201611.