After a lot of trial and error, I finally found the mistake. I noticed that I can load textures that are not located in the texture folder I specifically created for the TM file. This means the problem lies somewhere in the link between the (texture) folder I already created for the 2023 work file. Until now, I had only tried loading textures/material channels from that folder. But then I randomly tried loading an image from my Downloads folder—and it worked.