I've found the term Raubritter (German), which is translated to Robber-knight, and I was wondering - was it a popular thing in the 15th century? Were there many such knights? If they were tolerated by the appropriate king, why was that so? Are there any examples of famous, factual (meaning - they really lived) robber knights in the lands and times I'm interested in?
The modern sense is reasonably well known.
Wikipedia's entry for robber baron notes that the term robber knight is also used. In other words, robber knights certainly did exist. The wiki notes that they were a significant issue during the Hundred Years War which did cover parts of the 15th century.
Robber barons was an epithet that was assigned to those who preyed primarily on the river traffic on the Rhine. They became something of a virus during the interregnum of the Holy Roman Empire during the 13th century.
As highlighted in the above excerpt, robber barons became a real problem only when there was no real authority to question them. Furthermore, they holed themselves up in highly fortified castles or castle-towns which were sometimes on an island in the middle of the Rhine. They were consequently not an easy target to deal with and required a concerted effort to bring down.
The Baron of Rietberg appears to have been a famous robber baron as, according to Google, were those of Katzenelnbogen, Reichenstein, Sooneck, and Ehrenfels. It is unknown whether any of these robber barons were active during the 15th century.
Originally, most knights were robbers. That is to say in the "bad old days" known as the Dark Ages. This was a period when Europe was basically in chaos, central authority was distant or non-existent, the population was declining and losing wealth. In such a vacuum, power rested in the hands of LOCAL authorities. The invention of the stirrup gave the newly-stabilized armored knights mounted on horses a huge advantage over anyone that didn't have these advantages. Therefore, small bands of such knights roamed the countryside under a handful of leaders and robbed and terrorized the "locals" into submission.
What curbed this tendency was the rise of chivalry in the late Middle Ages (1300-1500). http://en.wikipedia.org/wiki/Chivalry This was a code of honor (or "ethics") that applied to knights. Specifically, they were supposed to fight only "equals" (other knights) and protect people weaker than themselves; especially women and children, and even unarmed men, particularly if "old." Also, they were supposed to submit to the authority of the king, which is to say that they were not supposed to kill or plunder unless ordered to do so.
The development of the code of chivalry was incubated by the rise of European populations and the restoration of central authority between 1000-1300. It received a further boost with the resulting development of art, philosophy, and science between 1300-1500 that ultimately led to the Renaissance, Put another way, "knighthood" now became a "profession" instead of a "vocation" because of these developments.
Although chivalry was developed in Germany, it gained earlier acceptance in the warmer, milder climate of France, where there was a strong central authority after the end of the 100 Years' War (1453) and to a lesser extent, in Western Germany along the Rhine. By 1500, it was established in these, more western, parts of Europe, but NOT in the colder, harsher climes of eastern Europe including Poland, Silesia, and East Germany (as of 1945-1991) where "central authority," economic development, and population growth was still quite weak. That's why you will hear stories of "robber knights" in these areas in 1540, and not in the more western, civilized parts of Europe, where this phenomenon had disappeared over the previous 500 years.