If u have reinstalled the application , try to clear Application Data and Cache Data once before trying.
You can also check Settings> Accounts > Office >... (3 vertical dots) >Remove account to start afresh
Check the Settings > Memory > Average memory use.
I have around 500-700MB free on my phone most of the time, about 30-40% free memory.
I'm not facing issues with Microsoft office applications.
I have the same problem with two different Nokia 5 handsets running 7.1.2. Disappointing to read that this issue remains two months on! No idea if this is a Nokia, Android 7.1.2 or Microsoft issue.