While I sympathise with you in needing a solution, I don’t see much wrong with the discussion. Debugging a software issue isn’t magic, it can be quite complicated, especially when there could be trouble to reproduce the issue … you did notice Threema works perfectly fine for another user using the same OS?
The bugtracker is the right place, though, and Fairphone staff did react, including a developer, now they’ll have to see what they can do. Professional help doesn’t automatically mean it’s blazingly fast, which can be frustrating for sure, but this is not a perfect world we’re living in.
Some brainstorming then, perhaps others will pitch in with more ideas …
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Apparently the other user is running Threema Version 4.53 Build 675 successfully.
Which version are you trying to run? -
Since background activity seems relevant and you encounter the issue with other Apps, too …
Is the battery saver on? (Settings - Battery - Battery Saver)
Can you remember whether perhaps Battery Manager asked whether it should restrict the Apps in question because it detected them as draining battery?
Are the Apps in question battery optimised? If so, it might be worth to change this to “Not optimised”. (Settings - Apps & notifications - See all […] apps - (select the App in question) - Advanced - Battery - Battery optimisation) -
Do you use an Antivirus solution on your phone? If so, it might be worth a try without it.
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I see you deactivated some things? Can you list them exactly and say how exactly you deactivated them (as there are several ways)?