I usually manage to request special meals without a problem (flying TATL with skyteam). Now ik booked a trip with one leg AA and one Ibera.
The problem is that they give me no seemingly no possibilty to request my special (vegetarian) meal on the respective websites. (AA doest't show the option, and IB doesn't process the request)

Now am I wondering whether this is something that can be solved. I do realise i am flying on the lowest possible fares, but i hope they don't exclude me from this service for that reason.

Is there anyone that has some hints?

I don't know about IB, but the resolution with AA is simple: They let you reserve special meals on any international flights (as well as US transcons in first -- they don't serve meals in coach within the US), with any fare you've booked (including awrd flights), but only over the phone. (They simply don't have the technology in place to reserve special meals online.) It has to be done at least 24 hours before the flight.

If you're based in Eruope, then, the quesation would simply "devolve" to how you would call about something like this from where you are. (I'm based in the US, and am an AAdvantage member, so I can simply call the number on the back of my AAdvantage card as-is from my home phone, and they can take care of the special meal request typically within a minute or two.)

Whoever sold you the ticket got a fee from you/commission from the airline. They are the ones to arrange for special meals: contact them!

I eat mostly based on paleo, low-carb avoiding gluten totally. What special meal would be best for me? And yeah talking about economy class here.

Thougth about gluten free meal but it seems to contain wheat which is a big no-no for me. I have enjoyed non-veg hindu meal over the years but now I'm bored with it (also there's gluten and wheat there at least sometimes) and want to try something else. Salads are okay with me as long as there would be some protein there.

I also enjoy getting the food first as I'm pretty slow eater especially in the plane.

Huh? Someone correct me if I'm wrong, but wouldn't a "gluten free meal that contains wheat" be like a "vegetarian meal with beef and chicken?"

It sounds to me like you're using the "special meal" option as an attempt to get something more to your liking than is offered in the standard meals. The special meals are designed to address people that have specific dietary needs due to medical or religious/moral requirements, not someone trying to follow the latest fad diet. Although anyone is free to order any of the special meals, consider that if one is not to your liking, the purpose of that meal being provided at extra cost to the airline is so that someone that has a specific need doesn't otherwise go with no food, not to provide a wider selection to all passengers.