Hello Eleni Constantinou,
Thank you for your detailed reply.
I will respond to your comments/questions/suggestion in order:
Question: Are you using Microsoft 365 or a Business account (*** Email address is removed for privacy ***/*** Email address is removed for privacy ***)? Or using personal account type such as Outlook.com, Hotmail.com, Live.com and so on? Or using other third-party POP/IMAPaccount in Outlook?
Answer: I am using a Gmail account. On both laptops, running Windows 10 and 11, there are no problems sending or receiving. The only difference is that Outlook on the laptop running Windows 11 does not sync folders unless they are manually opened.
Question: Outlook version on your Windows 10 and Windows 11 system?
Answer: Here are screenshots of that page for both laptops. First the laptop running Windows 10:

And here for the laptop running Windows 11:

Sorry that the screenshots are in German; the information should be clear, however.
(For future screenshots, I switched the user interface language to English.)
Both Outlook versions are part of my Microsoft 365 subscription.
The version on the Windows 10 laptop is Version 2407 and on Windows 11 is Version 2408.
Suggestion: Update Outlook.
Answer: Done. No change.
Suggestion: Cached Exchange mode / select Download shared folders
Answer: These options don't exist in my Outlook. When I select the account under File > Account Settings > Account Settings > and then click on Change, I do not get the option for More Settings as you said I should. Here is a screen shot:

Small window on left opens after I select File > Account Settings > Account Settings >
Small window on right opens when I then click on Change
Suggestion: Create a new profile.
Answer: That failed as well, and I am very baffled by that fact.
I went through the steps, entered the correct IMAP info and password and everything and got an error. See screenshot:

I tried it three times and double-checked the password every time. I also compared the POP and IMAP account setting to the existing profile. All correct, all the same, but I am unable to create a new profile. The old profile continues to function with the same info. See screenshot:

I am stumped.
Any more suggestions?
Thanks!
Christoph