How to avoid error in "SAGA Strahler order"-process when running it on a merged terrain model file

I am trying to create a catchment in QGIS 3.8 following this tutorial: https://www.youtube.com/watch?v=2fvy9xtAHc4

However, my terrain model is originally divided into 4 overlapping tif-files. I can follow the steps in the video if I only use one of the four tif-files, but I wish to merge the four terrain models into one and analyse the the whole area. I have therefor used Raster > Miscellaneous > Merge to merge the four tif-files into one tif-file. I set the input and output nodata-value to zero, and everything seems to be working fine. There are no error messages, and the output layer seems to be correct.

Then I do the "Fill sinks (wang & lui)" process on the merge terrain model, and still everything seems fine. There are no error messages, and the output layer seems to be correct.

Then, when I try to do the "SAGA Strahler order"-process, it seems like the execution finishes but there is no output from it and I get the following error:

Loading result layers
The following layers were not correctly generated < ul >< li > L:/temp/MFP/Elvenett.sdat</ li ></ ul > You can check the ‘Log Messages Panel’ in QGIS main window to find more information about the execution of the algorithm.

I have tried to run this operation many times now: in a local folder, in a path without any space characters, only with temporary files but I always get the same result. I haven’t found any similar problem online, except maybe this unsolved one (could be the same problem as me, but maybe not): Fill sinks and Strahler Order error in QGIS

EDIT:
I think this can have something to do with the terrain file size. I tried clipping the terrain model into a rectangle, and that magically did the work. At first I thought it had something to do with the shape of the terrain model, maybe it had to be shaped as a rectangle or another "regular" shape.

However, when I applied this theory to another terrain model I am working with, it did not help. I tried to clip out a very small and very irregular shape of that terrain model, and that did the trick.

So, is there a size limit to terrain models in the "SAGA Strahler order"-process? My terrain model, before cutting it, is "only" 150 MB. If this is too big, then that is a major limitation to this process.