There seems to be a new bug introduced with the latest 1.12 version.
Since I did not use it directly but used l5-swagger, which is using swagger-php which uses doctrine/annotations, I'm not exactly sure how to reproduce this easily.

What I can tell is that in line 1447 of the DocParser.php index "0" of the variable $positionalArguments is directly referenced, but the first set index in that variable at this point for me was "2".
This is what I saw when I var_exported the content right before it failed:

So this should be an easy fix I think, but I'm not deep enough in that library to be able to both create a test and fix this issue myself.

For now I reverted back to 1.11.2 and everything is working again.