Dreamfall ~ 3 glyphs
 

Supposed to open a portal.

Spoiler:

Follow the path to a wall adorned with three glowing notes. These must be pressed to match the tone heard when you activated the water wheel. From left to right press them in this order: 1,2,1,3 and a portal will appear.

Spoiler:

See 3 glowing glyphs on the wall. Press each one to hear the sound it makes.
Remember the tone heard when the waterwheel was activated. Reproduce that sound by pressing the glyphs in correct order.
Press left, middle, left and then right glyphs. A ring of fire opens up a door on the wall.

After stuckness, tried 2 walkthroughs (above).
Don't work at all...... I wonder why !!

I've had this call for help on 4 Forums, and one gave me a link to the Dreamfall Forum which led to a solution enabling me to continue the game !

All other suggestions unfortunately did not solve the problem... but I thank everybody who took time & energy to suggest solutions !!

It was mentioned (almost in passing on that link's thread) that reducing the antialiasing might help. I didn't believe that actually.
The game's video options include 5 degrees of Antialiasing, 4 degrees of Graphics Quality, and 3 degrees of Effects Quality.
Having a very powerful computer I had set all these 3 to maximum.

After the abovementioned hint, I set all 3 to minimum.
Lo & behold... everything goes as smooth as butter.
N.B. Not even any of the suggested available 'preliminaries' are needed (e.g. listening to the notes first; getting the game hint to recommend finding a tune; whatever !!). :)

Addendum.
Not so simple.
I've tried all sorts of combinations... I don't know why I bothered since I'm not a tester.
It works occasionally, but often not.

I am almost sure that there's a bug there... several other players have had an apparently random identical problem ???

Strange. I remember getting stuck on this, but checking the walkthrough resolved it for me. (Is it randomized?)

I'm almost certain that it's not randomised in the sense that I'm pretty sure that you always have to press the same sequence of glyphs --- left, middle, left, right.
N.B. You must have the glowing egg with you obviously... or you won't 'get' any glyphs to press !!

However, I do seem to succeed randomly and I have no idea why.
Returning to the same save well before this (simple) puzzle, most times I fail but twice I have succeeded.

This seems to be a weakness or a bug since several others have reported problems at this same point !