filter by taxonomy term = taxonomy term of referencing nodes I have a node type 'hotels'. These nodes have a reference to a vocabulary 'regions' and a field 'hotel_images'.

And I have a node type 'region_page' with also the reference to the vocabulary 'regions'. In these nodes I integrate a view with images of the field 'hotel_images'.

This view should be filtered by the 'regions'.

In case the node (type: region_page) is set to term 'xy' of the vocabulary 'regions' - only images of hotels for which the term 'xy' is also set should be displayed.

How can I do this?
If there is no specific requirement to use taxonomy terms to create the relationship between 'hotels' nodes and 'region_page' nodes, I recommend to use the Entity reference module instead.

In your use case, you likely want to attach an Entity reference field to the 'hotels' content type, that allows the content editor to point to a node of the content type 'region_page'.

If I understand it correctly, you already have a View attached to your 'region_page' content types' display. Assuming you're using the EVA: Entity Views Attachment module, you should add Content: Nid as your contextual filter in the particular View, create a relationship using Entity Reference: Referencing entity and then use this relationship for outputting the field 'hotel_images'.

In this way, when viewing a 'region_page' node, the View will show all images that are stored on 'hotels' nodes that have a reference to the particular 'region_page' node.