Sorting out the confusion: USA 161 and IGS 8R
The object which I photographed during the night of May 30-31 and which Björn Gimmle photographed from Sweden on April 22, turned out to be not USA 161 but, as Cees Bassa pointed out, another classified object we had "lost" in the winter blackout: IGS 8R (2013-002C), a Japanese radar imaging satellite launched early 2013.
So the image below actually shows IGS 8R flaring brightly, not USA 161 as I initially thought:
Luckily, Russell Eberst in Scotland observed the real USA 161 on June 1. Together with the observation by Leo on May 23, this means we do have an idea of the current orbit of USA 161 now, although further refinement through more observations is necessary. What is clear, is that USA 161 still is in the same orbital plane it was in when we lost it in August 2013. It's RAAN difference with the primary East plane KH-11 USA 224 is still 20 degrees, and it's orbit is sun-synchronous and about 385 x 393 km (these are approximate values which are subject to change, as the current orbit is preliminary and needs some refinement with more observations). The current KH11 Keyhole/CRYSTAL constellation now looks like this:
The short-lived confusion of yesterday could arise because both objects (IGS 8R and USA 161) currently move in a similar orbital plane. This can be seen in the image below, where the IGS 8R orbit is yellow and the USA 161 orbit is light grey:
This is the kind of confusion that can arise when multiple objects who's orbit have not been recently updated, move in a similar orbital plane. It does not only happen to us amateur trackers: even the professionals at JSpOC sometimes confuse objects.
Actually, this situation ended positive with a double recovery: that of USA 161, and that of IGS 8R.