I've been having an issue with the SAGA functions for a while now, I believe since upgrading to one of the early 3.x versions. No matter which function I use they all fail with the same error (screenshot below). GRASS and GDAL functions work fine. The way our network is set up, I'm frequently wrestling with extremely long file paths, but I still get the error when testing with a copy on the desktop. In writing this up I have been able to get it to work after running Fix Geometries on some files but not on others, and that is the first time I've seen a SAGA function succeed in months. None of the files I'm working with are large, and I get the error regardless of apparent data quality of source. All the data I'm working with used to have no issue with the SAGA commands up until a few months ago.
I've been having this issue across various 3.x versions, using Windows 10 64bit. I've been using QGIS for ~2 years now and have not seen these issues up until maybe six months ago. I'm now using the 'QGIS 3.6.0 with GRASS 7.6.0' installed using the standalone installer. I've been running more attempts than I normally do as I try to gather info for this post, and am seeing intermittent success instead of consistent failure. Seeing that people have been running the commands fine, I wanted to try again and generate some better error logs for folks to look at. I opened an new untitled QGIS project in QGIS 3.6.0 with GRASS 7.6.0. Brought in a point file of address points, a polygon file of property parcels and a line file of roads. Ran a 10 ft buffer using SAGA on each of them. It failed twice in a row on the points, succeeded on the polygons and lines. I then tried running Thiessen polygons on the points, another command that has produced failures for me -- this was just to see if could get an error log from a different SAGA command, since I've been seeing it across multiple commands. The Thiessen polygons failed as well. It failed again after running QGIS 'Fix Geometries' command on the point file. I then opened QGIS 3.6.0 without GRASS, just to see, ran the same SAGA buffer command on the same point file and it succeeded(!). I then started a new project in QGIS 3.6.0 with GRASS 7.6.0, ran the same 10 ft SAGA buffer on the same point file and it also succeeded(!??). I then tried running SAGA thiessen polygons on the same point file and it failed again.
I saved most of the logs from these attempts and put them into Google drive: https://drive.google.com/drive/folders/1fOWhkdfXLYRDvh9OmjJzZlJ5K0XEaQNe?usp=sharing