Is this rust fungus on leaves of my Pahphiopedilum Harrisianum

[estación seca]

It may be an older leaf dropping naturally. The rest of the plant looks healthy. If it does not spread to other leaves I would not worry. Rust would rarely or never be seen inside a home in North America but I don't know about Europe.

Do you know when it was last repotted? Most Paphs should be repotted every 1-2 years because the medium breaks down rapidly from being constantly moist.