Hi Guido,
It sounds like the issues you are having are related to that device specifically. I have tried to replicate your issue using v2.5.44 on similar devices such as Samsung S6 Android 6.01, Samsung S8 Android 7.0, and even older Samsung S3 Android 4.4, however we do not have a Samsung S6 Android 7.0 to match your device.
I am able to search, download and open surveys created in Connect or Web Designer, as well as opening surveys via URL scheme. Also tested surveys with hundreds of questions and other surveys with Inbox enabled, attachments and media folder files etc. The app is working as expected on the above android devices tested. What is the format of the URL scheme you are using to open the survey in the app, and are you passing in any parameters?
Are you signed into the app (AGOL account) with the same user that downloaded the surveys to device? Have you tried to sign out and back in and then open a survey. Alternatively, sign out, close the app fully from recently open menu, restart device and then open the app and sign in again.
If the above does not work, we need to check the following. How much free storage is available? We have seen issues on other devices when the storage is close to being full (<100mb), the app will crash or not respond, along with other apps on the device. What other apps are installed on the device, for instance do you run any special virus apps or encryption apps, MDM profiles etc. Do any other apps on the device have strange behaviour or crash regularly?
As a last resort are you able to hard reset the device and setup the OS and Survey123 app again? If so I would be interested to know if this resolves the issue.
Regards,
Phil.