When I scan on safari on mobile, it will barely process and then show this screen. I’ve tried 1 picture at a time and it still doesn’t work. Tried on firefox and chrome as well.
The scanner works normally for almost all people. It has been reported before that it didn’t work on certain phones (or very sporadically). I suspect it has something to do with not having enough available memory on the phone to parse the image on the device. It’s a bit hard to solve (if at all possible) since none of the devs can reproduce the issue, unfortunately.
I can confirm that it sporadically happens on a Samsung Galaxy S22. Usually it works well the first few times and after a while the above issue appears. Force closing the app and re-opening it fix it in my case (I’ve added the page as app). I’m using the Edge browser.
I hope this helps and happy to share more details in case someone want to look into it.
It also happens for me on Poco F3. Reloading the page (up to 3-4 times) fixes the problem.
Weird. On iPhone 15 Pro, should have plenty of memory. Tried forcing the app, trying different browsers, restarting phone, logging in and out, using the page as an app.
My guess is that it isn’t a hardware issue, but that there is a bug that allows you to upload the images before everything (like the recognition model) is actually loaded.
For me specifically every single time i want to scan card on phone, the first time i load them i always (no matter what), get an error, after that i go to the overview page, return to the scan page and proceed a second time, 90% of the time it works at the second try while the remaining 10% require at least 3 tries, but the thing i’m sure of is that the first time i scan, it never works, so at this point, i’m just used to do it 2 times at least XD.
Btw my phone is a galaxy a23 5g, so pretty old model, so on my end the issue most probably regard memory or the phone model as you mentioned.
Pretty much exactly what I experience on my Samsung Galaxy S22+. I pull down to refresh the page and keep trying until the scan actually works. Usually once the scan finally takes, subsequent scans are successful as well, it just always takes a few attempts to get it going. Reliably unreliable 
What happens if it fails, then you click on the overview tab on the top and click back to the scan tab and try again. So without refreshing the page. Does that change anything?
I just tried another scan on mobile. It failed, I tapped “Overview“, then tapped “Scan“ again, submitted my images again, and then it worked. Sample size of one though, I’ll continue trying and see if that consistently produces a positive result.