After this question was closed over at RPG Stack Exchange, I'm still left wanting a comprehensive answer. My question, again, is this: What is a realistic estimate of the weight an 11th to 13th century backpack could have held without breaking?
The historical answer is pretty much the same as the answer that you got on RPG - it depends on the type and construction of the pack. These ranged from simple cloth bags designed to be worn on the back, such as this one from one of the panels at the church of St.Wolfgang in South Tirol, Austria...
...to full on back boards and baskets made from wood and wicker, like this example from The Mendel Housebook:
Keep in mind that these would be no means have been the most common way of carrying things in the 11th through 13th century except for things that were too heavy to comfortably carry in other ways. Much more common was a simple over the shoulder satchel like the one illustrated in the frescos at the Church of St. Nicolas in Travant, France:
So basically, a medieval backpack's carrying capacity would be determined by the same factors as they are now - method of construction, materials, and construction quality. Ultimately, the capacity comes down to how much the person carrying it can lift.
These guys in Nepal carry loads up to about 85kg per porter. The "backpacks" are actually wooden frames for weaved baskets, held together with ropes. This technology was already widely available much before medieval times. Any specific piece of luggage, then as now, had its own breaking point, but since this is way before standardized production in high volume, there is no such thing as a well-defined "average capacity" for a medieval backpack.
As other data points, consider the amount of equipment transported by foot soldiers when marching, in organized armies in history. It has been discussed here already; roughly, an individual infantryman can be expected to carry 30kg and still walk his 20 km a day (distance covered depends much more on the quality of roads than the load). It helps when some of the load is hung before the torso instead of the back. This already held during Antiquity.
RPG games with a "medieval" setting balance their operational parameters, such as maximum backpack capacity, to strike a compromise between gaming experience (players must be able to carry enough loot not to be frustrated, but should still have to manage their carrying capacity) and some dose of "realism" (more based on the player's Hollywood-fuelled expectancies than history books), in as much as you can get realism from a game where you can also punch tyrannosaurs.
A maybe more important question is how much a human can carry without compromising his ability to efficiently fight with blades or other medieval-era weapons. This is not an easy question. A 15 kg mail armour is tolerable and won't impede fighting abilities too much, if you use appropriate fighting techniques (i.e. don't try to do some acrobatic 17th-century fencing); a strong sturdy belt is critical since it helps putting the load on the hip bones. A 15 kg backpack protruding from your back will be much more of a problem, since it will put you off-balance. Shape is very important too: a fighter needs room for his elbows (this argument has been used as a possible explanation for the figure-of-eight shields from Mycenaean Greece: the peculiar frame would have allowed a fighter to use a pike or spear while carrying the shield on his back).
If I were to design a RPG, I would put a hard limit for unencumbered actions at around 15 kg, with worn body armour counting for only half its actual weight against that limit.
Given that they had canvas and leather, I don't think there were any technological caps on the carry weight of a backpack.
Better to look at what a human can carry comfortably for a prolonged time, which seems to be 20-30% body weight if you browse a hikers' forum. That's with a modern backpack, of course. I had a look and couldn't find whether backpacks were invented back then, so you may want to take into account less ergonomic means of carrying.