Thanks very much for your input. All plants are in the same medium (this one:
https://www.bio-kultura.nl/Biologische-Potgrond), which makes me wonder why only one should be suffering. Maybe, if you are correct, I should be wary that rot could be developing in the other plants too.
Thanks for the helpful pointers regarding improving soil drainage, I will implement for future repottings. I had assumed that by using pots with drainage holes that I was safe from root rot (especially in this hot weather).
Unfortunately I'm not able to inspect the roots for a few weeks, but if it is still alive when I am back I will certainly share the results. Any other pointers or ideas much appreciated!