no valid user IDs after changing key expiration time

[flapflap]

Ludwig Hügelschäfer:
> On 22.07.15 16:36, flapflap wrote:
> 
>> Should I be worried by the warning or is this normal behaviour?
> 
> You should set ultimate ownertrust on your own key after
> (re-)importing. Then it will become valid again.

My key still looked/looks valid, even without changing the ownertrust
(in that case its ownertrust is just shown as unknown).  All UIDs are
present.

What seems strange to me is that I do not see the warning message when I
import the previous version of my key (exported about a year ago).
The warning only appears when I change the expiration time of that key,
export it, delete it, and then re-import it (including secret key).

~flapflap