Any ideas about this detail? (Middle right rosette).

[cvetkakocj@rogers.com]

I don't recall having seen any explanations for this. Has anyone ever seen anything like this elsewhere? It looks like two "tails" coming out of a central point, but I have no idea what the intention may have been. I'm also not sure if the "tails" angle is a good one, since the outer lines seem to make two half circles towards the cloud bands. 

Conceptually, we can probably assume this scene takes place either in the sky or in the heavenly realm (or both). It is probably something you cannot touch when you are on earth, at least that would be the meaning of the cloud band in standard iconography. They also used a special method of painting, applying curved dabs of blue paint, maybe suggestive of clouds?

There is also a strange detail of a couple of lines bottom left on the inner circle.

Hi, Koen, I believe this is very much in line with your idea of Revelation, except that the author of the VM had a different understanding of the Revelation. He regarded Jesus as the Word of God.

Before that circle, I noticed two pathways that contain the images of two triangles touching the circle in the middle. The image looks like two Glagolitic letters, composed of a circle and a triangle, one standing for SLOVO (a letter or a word), and the other for IŽA (house). It has been proposed that the two of these letters next to each other represent the initial for a deity in the medieval Old Church Slavonic letter. In my opinion, the initial stands for the 'house of divine words', assuming that the Bible is where the Holy Words are 'housed'. 
    The Circle is an ancient symbol for God, and the triangle is a symbol for the Trinity. In one picture, the triangles contain two images: an image of lighted candle in a candle stick, which looks like the symbol of Waldensians, and the other looks like the Prince's Stone, where Carinthians democratically elected their dukes. The ritual was conducted in Carinthian (Slovenian) language which was spoken in ancient Carniola, Slavonia, Carinthia, and parts of present day Switzerland and northern Italy. The ritual in Slovenian language was conducted up to the 1415.

The second 'pathway' shaped with two triangles and a circle contains a church that has been identified by some as Maria Laach Abbey. This was a centre of religion and culture, the earthly house of books, particularly holy and mystical books. 
When I first attempted to interpret the image you are wondering about, I was mostly impressed by the cloud band that encircles it. I could not relate the picture to any realistic image, but since the cloud band (as well as the blue colour) is suggestive of a spiritual reality, I regarded that circle as an allusion to a mystical religious experience, a mystical revelation, and the image in a circle as a monstrance. A Monstrance is a church vessel where the Holy Host is displayed for adoration. This is symbolic representation of something invisible, indescribable: the mystical incarnation of Jesus, as a Divine Logos. 
In the mystical experience, the human and the divine touch for a brief moment in what the Church calls 'the earthly mystical union with God'. Unlike the Church symbolism, the mystical symbols are God and Trinity, on Earth as in Heaven. For a mystic, in his altered state of mind, nothing else exists, but the inner words of the Revelation.