Short summary: the two planes were very close, “200 feet vertical and 0.2nm horizontal,” and according to the article the main reason the incident happened is due to miscommunication and oversaturated communication channels.
Initially the Wizz Air plane was supposed to descend to FL300, but missed the first instruction, and the second one was incorrectly given and confirmed (read back by the crew means confirmed?) as FL200. The Air Europa flight was at FL290, and the Wizz Air plane got to FL292 (200 feet from FL290) before being instructed to ascend. The cause of the miscommunication was that the frequency used to communicate from the air traffic control to the planes was saturated and many communications collided, meaning multiple communications happened at the same time and were impossible to differentiate, necessitating repeating.
Edit: wasn’t misheard, instruction was incorrectly given.
Initially the Wizz Air plane was supposed to descend to FL300, but missed the first instruction, and the second one was incorrectly given and confirmed (read back by the crew means confirmed?) as FL200. The Air Europa flight was at FL290, and the Wizz Air plane got to FL292 (200 feet from FL290) before being instructed to ascend. The cause of the miscommunication was that the frequency used to communicate from the air traffic control to the planes was saturated and many communications collided, meaning multiple communications happened at the same time and were impossible to differentiate, necessitating repeating.
Edit: wasn’t misheard, instruction was incorrectly given.