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It would help if you mentioned the Andoid phone you are using.
The first thing to do would be to clear your application cache and application data. This is done by going into your applications and clear cache and data on the applications causing a problem.
However, if you have a phone with a limited amout of storage, and have too many applications open this may be causing your applications to immediately close and you may need to free up some space on your phone by deleting unwanted applications.
It would also be worth checking you have the latest updates for the applications, this can be done by accessing the Play Store and going into your installed applications. If you click on your applications, you will be able to check if there is an update available.
If your factory resetting and then restoring your info then if the issue is in the back up your just re introducing the issue @Andreodi
What I would suggest is either put the phone in safe mode which turns off all your downloaded apps and leaves the stock ones alone which can help to determine if a rogue downloaded app is causing this situation if it stops and then take it out of safe mode and start deleting your mkst recent downloaded apps one at a time or factory reset but do not use your back up.
Set it up as new to see if the issue continues.
Ensure your firmware is up to date.
If it does then a Vodafone High street Store can send it off for a diagnostic.
Current Phone >
Samsung Z Fold³ 5G.
Previous Phone >
Samsung Note 20 Ultra 5G - SM9860.
If you have done a factory reset, this should have cleared everything. However, if you did a back up, you will still have the same applications on the phone.
At a guess, you have had the phone for a whilst and will now be out of the 2 year warranty. If that is correct any repair would be chargeable.
Compared to the latest phone available, the Ultra 6 only has 16 GB internal and 2GB RAM, you may find just freeing up some space might stop the applications from closing. As well as deleting any unused apps, try deleting any old text message conversations and clearing your call list, this may be all that is needed.
The phone is still in warranty, take it into a Vodafone Store and they will be able to take a look and if needed, arrange to send the phone away for repair.
You have a handset issue, the only people who can look at your phone are in a store. You need to go back to the store and explain your problem. I know this is something you have already done but you are going to need to be persistent.