Could this be because of the battery savings option on the iPhone? That your iPhone doesn’t allow apps to run in the background? 

Thank you so much, I have already checked that, the saving mode is not activated in the settings

Hi @KarlaP, thanks for your message.
Apps can sometimes crash when storage space is not adequate.
You will need to free up your storage space by removing unwanted apps and files.
Go to Settings -> Apps to uninstall unwanted apps and games.
Please let me know if this solved the problem.

Thank you so much for your replay. I tried to delete a lot of it, but still the same