Is bringing tea from coca leaves to Europe illegal?

This was covered in the early 20th century by some other laws, but in 1961, the Single Convention on Narcotic Drugs was established by the UN and as of May 2013, the Single Convention has 184 state parties. The Holy See plus all members of the UN are state parties, with the exception of Afghanistan, Chad, East Timor, Equatorial Guinea, Kiribati, Nauru, Samoa, South Sudan, Tuvalu, and Vanuatu.

The coca leaf is listed on Schedule I of the 1961 Single Convention together with cocaine and heroin.

The Convention determined that "The Parties shall so far as possible enforce the uprooting of all coca bushes which grow wild. They shall destroy the coca bushes if illegally cultivated" (Article 26), and that, "Coca leaf chewing must be abolished within twenty-five years from the coming into force of this Convention" (Article 49, 2.e).

And why?

The historic rationale for international prohibition of coca leaf in the 1961 Single Convention comes from "The Commission of Enquiry on the Coca Leaf study" published in 1950. It was requested of the United Nations by the permanent representative of Peru, and was prepared by a commission that visited Bolivia and Peru briefly in 1949 to "investigate the effects of chewing the coca leaf and the possibilities of limiting its production and controlling its distribution." It concluded that the effects of chewing coca leaves were negative, even though chewing coca was defined as a habit, not an addiction.

Peru and Bolivia, however, have made an amendment, which you can read further about on that page, which is how you're able to have it there. There are some other exceptions supported by some countries, but basically - in Europe - no, it's not permitted...yet.

It may be illegal but the authorities turn a blind eye to it. I declared a large packet of coca tea to the German customs at Düsseldorf airport, they waved me through and looked annoyed that I had even bothered to ask them about it.

I think the answer may be Yes, and Yes.

That is - yes, it is probably illegal. Yes, you can probably bring some as a souvenir. Yes, those answers contradict each other.

Coca leaf is banned, under the narcotics laws, in most every country except a very few, including Europe and USA in the list of countires where it is banned (as mentioned in Mark Mayo's well researched answer). Coca leaf was banned, it looks like, specifically to interfere with people chewing the leaf as a habit. Of course, the law doesn't care to specify a difference between leaves intended for tea purposes, rather than coca-chewing purposes, since the leaves themselves will be the same - but that emphasis on the chewing as the reason for the ban might be the reason why some sites claim teabags would be let through, while loose-leaf might not be.

On the other hand, coca leaf tea is available for sale in the US (I realize your question was Europe, but the same set of laws are in play). I saw a few listings, on amazon and ebay, so someone is importing it while under those laws, in salable quantities. I think in practice the restriction is working like it has to do with those authorized to import and distribute, rather than a blanket ban. We have 1690's anecdote that some customs will allow it through. It may be that the tea is low enough on the priority list for individual customs people, that it isn't reliably recognized as banned. It might be that the presence of legitimate sources for the tea, obscures whether your souvenir packet was from one of those legit sources. It may also have to do with the totally legit cocoa tea (cocoa nibs are becoming popular additions to some tea mixes) and whether the legitimacy of the two might be unclear in some customs officer's minds.

So, if you pick up a small quantity of mate de coca, you have a fair chance of keeping it, and another fair chance (if it is caught) that it will simply be discarded, with no extra penalty to you. A grocery store box of teabags looks like something a person might not have thought about - even if you really did - so they will probably treat it like a mistake unless you've got a really suspicious quantity or other red flags. Obviously, your chances are better with a small quantity obviously intended for personal use rather than resalable quantities (or, hoarding for an extended timeline of use quantities, which can look the same). Your chances are considerably better with neatly labeled, grocery-store-sealed boxes of teabags rather than a quantity of loose leaf or raw coca leaves in a hand-labeled bag.