I have just received a "Notice from BOINC" that says

Collatz Conjecture: Notice from BOINC
This project is using an old URL. When convenient, remove the project, then add https://boinc.thesonntags.com/collatz/

I tried this on 1 laptop and it worked fine
I tried this on a 2nd laptop and the [Remove] button is greyed out.
I am unable to remove the project.

Before removing everything and start from scratch on this laptop, any advice ?

When the Remove button is disabled, you have added the project through different means, most probably an account manager such as BOINC Account Manager (BAM!) or GridRepublic. You'll have to remove the project through the account manager, before you can add it again (be it through the account manager or through BOINC Manager).

You can check if you're using an account manager by looking in the Tools menu for "Synchronize with X", where X tells you the AM you're using.
If this is an older BOINC, the Sync place may differ.

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