Indeed I run the query a few times. I will do the tests after I will clean-up the buffers. On the machine it was only me, no other activity could interfere. For instance I put this task in background, some others came along with higher priority and I found some workarounds:

1. declare @var int = 'value'

select * from userdata where 'key' = @var this does an index seek

2. the index on 'key' shloud be modified to contain the other fields with include

3. the forceindex

Thnak you for your replys and hints!