Hello again lock042. Thank you for your quick reply.
Although I will try the first suggestion, I think the main issue is as I wrote, that Siril thinks that I have that 1000 file limitation.
None of the other stacking apps show that issue. I used the same SER videos with an older version of Autostakkert, in a Wine type wrapper, and it worked just fine.
Also as a feature request, for a future version of Siril, can the SER videos save the stacks as one tiff or fit file, instead of one for each of the SER input videos? That is how it worked when I loaded just two or three SER videos.
I then had to re-stack the two or three stack files to get just one final file.
If I am doing something wrong in the workflow, it may be because the info in the user manual is quite lacking, for processing SER videos. That area needs to be expanded with steps and examples for each module, such as is the case for processing images.
Thank you again.