Pull out the graphics card and reboot. It should still connect.
(If you have an internal graphics adapter, you could use that.)

Then put the graphics card back in. I have never had much trouble getting BOINC to change.

EDIT: For that matter, just try installing VirtualBox. That should easily confuse it.
https://www.cpdn.org/show_host_detail.php?hostid=1497371
https://www.cpdn.org/show_host_detail.php?hostid=1482424

Pull out the graphics card and reboot. It should still connect.
(If you have an internal graphics adapter, you could use that.)

Then put the graphics card back in. I have never had much trouble getting BOINC to change.

EDIT: For that matter, just try installing VirtualBox. That should easily confuse it.
https://www.cpdn.org/show_host_detail.php?hostid=1497371
https://www.cpdn.org/show_host_detail.php?hostid=1482424

Ok, I could just swap the graphics cards between the two systems, the Ryzen do not have on board graphics.

Changing computer name is not one of the triggers used to get a new user computer ID, basically BOINC ignores the name you give to a computer.
A change in operating system, bigger than just a Linux version change, but a more substantial type (Linux to Windows) will get you a new computer ID. Changing Linux family may count, but I'm not sure about that.
BIG hardware changes (e.g. changing from an Intel to an AMD CPU) will almost certainly get you a new computer ID.

There have been times when changing BOINC version has triggered a mass computer ID change, but that was very much the luck of the draw.

edit to add:
Linux kernel version sometimes works, but sometimes doesn't.

Thanks, I have a spare disk with mint and another with Debian I could try but nothing with Windows on

Hmm. I’m on Ubuntu 20.04 and the next update is 20.10 which, as far as I understand, gives problems with Boinc so I’d rather not update from the LTS version yet.

I have a fresh install of 20.10 on both my Ryzen and the old Laptop and both seem to be working OK with BOINC.

I have compiled from source however rather than using the version supplied by repository.

I’ll try to find out why I thought there was a problem

I did notice, however, that no alt platforms was still set from the days when I was getting 32 bit apps from ?WCG? that were crashing. I’ve reset it just in case.

It was worth a try, made no difference though.

I DO NOT BELIEVE IT,

After booting the new machine (which has never succeeded in downloading any CP tasks) using several disks holding various distros which gave various problems that stopped me running Boinc in the short time I had available, I reverted to its standard configuration and within about half an hour it downloaded a CP task followed by another one about 6 hours later.

Whether the above change has finally registered (and, yes, this time I did reread config file after making the change) I don’t know but it’s the only significant change I can think of.

In the morning I’ll reboot the older machine and see if that has the same effect.

I did notice, however, that no alt platforms was still set from the days when I was getting 32 bit apps from ?WCG? that were crashing. I’ve reset it just in case.

It was worth a try, made no difference though.

I DO NOT BELIEVE IT,

After booting the new machine (which has never succeeded in downloading any CP tasks) using several disks holding various distros which gave various problems that stopped me running Boinc in the short time I had available, I reverted to its standard configuration and within about half an hour it downloaded a CP task followed by another one about 6 hours later.

Whether the above change has finally registered (and, yes, this time I did reread config file after making the change) I don’t know but it’s the only significant change I can think of.

In the morning I’ll reboot the older machine and see if that has the same effect.

I think that confirms it, within an hour of rebooting I have work.

So, changing no_alt_platform from 1 to 0 followed by a reboot appears to have fixed the fault.