What should I study? My ELO is 2050. I had not played chess for 4 years until I moved to Amsterdam. In Amsterdam I joined a chess club which has many strong players. Every Tuesday we play 90min + 15sec incremental games each other.(I played 6 games)

I noticed that I could not calculate well enough for the last 3 games which I lost. Also I do not have solid opening knowledge. I started to study tactics from chesstempo. Am I on the right track?
Whenever I haven't played for a while I go through a book with 200 mate in two exercises. Easy stuff, but quite a lot, just to get into gear again. Chesstempo will serve a similar function for you. 

But that is just to shake off the rust and not blunder too much. To get back to the playing strength from four years ago or to become even stronger, you should rather train calculation instead of tactics. I.e. not just combinatorial vision but the systematic calculation of long lines. The Dvoretzky books have exercises for that, but those are pretty difficult.

The main point is that in my experience servers on which you solve timed problems aren't really suited to improve calculation. Even just picking random positions from these 10000 puzzles and calculating them like you would in a game is a better way.

You should also reacquaint yourself with your openings, by looking up your old repertoire lines and then play some internet blitz to get a feel for the variations again.

You are in the enviable position of living in Amsterdam so you don't really have to spend money on books yet. The Max Euwe Centrum has a library of chess books in many languages, entry is free and you'll find the newest books on every possible opening and avenue of chess improvement.