Outlook on PC won't login to personal account like it used to do

Windows 10 v21h1
Office 365 shared from me when she got a new computer back in March.
Everything used to work beautifully.

Now using the downloaded Outlook programme, she gets "You can't sign in here with a personal account"

Then using outlook.com : "it looks like this email address is being used with more than one account from Microsoft. which one do you want to use? work or school or personal"

BUT SHE ONLY HAS THE ONE ACCOUNT, WHICH IS PERSONAL.

Historically on her old laptop she used Windows LiveMail to access her TalkTalk and Gmail accounts, and had a Windows login account through her "@TalkTalk" email address. That is the account I used to allocate the Office 365 "seat". Using livemail she had many folders into which emails were sorted - but using POP not IMAP so not replicated in her TalkTalk account. Nor were her contacts.

You can't get livemail any more and she didn't like windows mail - too fancy and too spread out. So I set up Outlook, imported everything from livemail, and it worked. It also created the folders into her TalkTalk account.
Since then she has been happily using Windows, Outlook and syncing through her OneDrive. No problems at all for 4 months.
A month ago, when she opened Outlook she got the first message. There is no way to get past this as she has no work or school account.

• She has to close Outlook which gives a procession of out of memory messages
• Why, after months of working properly?

I tried to help her, and to ensure all was well, asked her to log in to live.com in her browser. That gave the second warning about work or school.

• What has work or school got to do with anything? She is a long retired individual and has never had a work or school account.
  
• If she tries to login using the top button (Work or school) it tells her that there is no account associated with that email address! We've tried all of them.
  
• As the Microsoft365 manager, I have no option to even see what her account email is! In the Family Sharing page it shows me her name but not the associated email.
  
• If she uses the bottom button to login using her personal account, it works.
  
• BUT IT WON'T WORK IN OUTLOOK on her computer, as she doesn't get that option.
  
• She just wants to login to Outlook on computer using her private account name, like she did on Tuesday and for the past 4 months.
  NB I am sharing with 2 other family members, and they have had no such issues.

What have we tried:

• Posting on Microsoft community. That offered a "solution" that required being able to login to the Outlook programme which she cannot do.
• Repaired Office 365.
• Close Outlook. If I look in C:\Users\<hername>\AppData\Local\Microsoft\Outlook it shows me three ".ost" accounts - named for her actual talktalk email which is large, her gmail which is smaller, and the Microsoft account which is very small (there were only ever some test emails in it). Renamed folder to OLDOutlook. Restart Outlook. Fail.
• Add a new "@outlook" email address to her Microsoft account, and change her primary Microsoft account alias to that (from talktalk). Try again. Fail.
• Uninstall Office 365. Rename Outlook folder. Restart computer. Download and install Office 365. Try again. Fail.
• Scream. Didn't really help.
• Using Office 365 Word or Excel opens them happily into her account.
• I've even removed her from sharing of Office 365, and reinvited her using her (now primary) "@outlook.com" address.

Currently, she is using Firefox to access her emails using webmail.


Is there anything reasonable that we can do to fix it so she can reinstall and use Outlook?

Sorry you didn't read it because I covered those points. She has no school or work account or relationship. She's only using a family invite from my personal Microsoft 365 account.

It is most confusing. I'm tearing my hair out.
The second image is from outlook on the web - Microsoft seems to think that she has a work or school account! It insisted on offering that login. Then it tells her she has no work or school account in any of her email addresses.
Yes, the Outlook on her computer was downloaded from her logged-in account on Microsoft 365 family. It comes as part of the full 365 package which includes Excel and Word etc.
Essentially we've given up on trying to use outlook. I've got her set up using webmail.

Thank you for trying to help, and I appreciate that what you describe is how it should work.
That is why I'm so frustrated and cannot understand what is actually happening.
I am the owner of the Microsoft 365 family subscription. My use is personal, for the family. I shared it with her. I have no admin rights over her account. Its not a child's account which might have restrictions. Her use is personal. Outlook worked perfectly for her for several months. I've changed nothing. She's changed nothing.
Overnight Outlook 365 on her laptop decided that she couldn't sign in with her personal account that she had done the day before.

Fellwalker said:

• As the Microsoft365 manager, I have no option to even see what her account email is! In the Family Sharing page it shows me her name but not the associated email.

If you click "Add or manage Microsoft family members" below her name, it will show you her email address.

(I just checked this for my wife who shares my Microsoft 365 Family subscription.)

She has no work or school account! She's 86. Last time she worked, computers were the size of a house and she wasn't involved with them. Last time she was at school, a computer was an abacus.
She never had office until I set up her new laptop for her and downloaded the Microsoft 365 suite using the link from my invite.
Sorry for any confusion. Microsoft365 was Office 365 and you always downloaded the individual programmes but now get the full suite in a package.
Yes I have since found and confirmed that the account is shared using her personal email address.