I'm facing the same problem for many months already. Contacted Boxcryptor support multiple times with many examples but they couldn't solve it. For me this is a key feature and one reason why I'm paying for Boxcryptor.
My phone is a Samsung Galaxy S9+ running stock Android 9. I tried every single troubleshooting idea and even different Android phones. But Boxcryptor continues to throw upload errors and more worrisome damages the image files. Every other image being automatically uploaded becomes corrupted and only displays half of it when opening it on another Boxcryptor client.
I consider that problem pretty serious and can only wonder why Boxcryptor is solving it for many months already.