The files that you have trying to combine, are those files are too large in size? Or what is the number on the files you are trying to combine at once?
If you are trying the conversion online then you may use a different web browser- https://cloud.acrobat.com/combinepdf and check.
For testing purpose, select an another set of the files and try combining them and see if that works.
If you are using the desktop application Adobe Acrobat Reader DC and you use the combine tool, try to clear the cache and cookies of the browser before using. .
you may check the related discussions- combining files error (Combine Files)
Hope this helps.
I am having the same problem and this started with the last update. I have tried smaller files, different files and different browsers and still the same error with no details???