Thank for your patience.
I checked my PHP-SDK-Version and it is 2.6.0, according to composer.json.
In fact, if I use another file for testing (e.g. the source you used in your own tests), annotation runs without the error message i received with my test file. So maybe it is the specific file which causes the trouble (or the fact I used it dozens times in the past - some relicts in the system?)
So I can state: it works in general! The only thing left ist the missing Thumbnails in the annotations-tool (even with your test file!)