The message indicates that a server-side error occurred during the update. The reason for the error should be visible in the error log. You can find it in your hosting area. If necessary, the support of your hosting can help you.
Alternatively, you could also enable debugging in WordPress. How to do this is described here: https://ww.wp.xz.cn/support/article/debugging-in-wordpress/ – also here the support of your hosting can help you if necessary to edit the file accordingly.
In both cases you should be able to see what the cause is in the logfile.
Thanks.
My host provides for two Log File Downloads: View Access Log and View Error Log.
I assume the latter is the relevant one. I ran the 6.1 upgrade routine again and immediately downloaded the logs.
The last minute of the Error Log is:
fhithich.uk [Wed Nov 09 20:17:44 2022] [error] [client 144.76.68.17:0] PHP: syntax error, unexpected '(' in /home/cluster-sites/2/f/fhithich.uk/public_html/php.ini on line 1
fhithich.uk [Wed Nov 09 20:17:55 2022] [error] [client 87.75.112.183:0] PHP: syntax error, unexpected '(' in /home/cluster-sites/2/f/fhithich.uk/public_html/php.ini on line 1
fhithich.uk [Wed Nov 09 20:17:57 2022] [error] [client 35.245.82.188:0] PHP: syntax error, unexpected '(' in /home/cluster-sites/2/f/fhithich.uk/public_html/php.ini on line 1
fhithich.uk [Wed Nov 09 20:18:07 2022] [error] [client 87.75.112.183:0] PHP: syntax error, unexpected '(' in /home/cluster-sites/2/f/fhithich.uk/public_html/php.ini on line 1
fhithich.uk [Wed Nov 09 20:18:14 2022] [error] [client 87.75.112.183:0] PHP: syntax error, unexpected '(' in /home/cluster-sites/2/f/fhithich.uk/public_html/php.ini on line 1
fhithich.uk [Wed Nov 09 20:18:14 2022] [error] [client 54.36.148.2:0] PHP: syntax error, unexpected '(' in /home/cluster-sites/2/f/fhithich.uk/public_html/php.ini on line 1
fhithich.uk [Wed Nov 09 20:18:16 2022] [error] [client 10.16.12.8:0] PHP: syntax error, unexpected '(' in /home/cluster-sites/2/f/fhithich.uk/public_html/php.ini on line 1
fhithich.uk [Wed Nov 09 20:18:18 2022] [error] [client 10.16.12.8:0] PHP: syntax error, unexpected '(' in /home/cluster-sites/2/f/fhithich.uk/public_html/php.ini on line 1
fhithich.uk [Wed Nov 09 20:18:18 2022] [error] [client 10.16.12.8:0] PHP: syntax error, unexpected '(' in /home/cluster-sites/2/f/fhithich.uk/public_html/php.ini on line 1
fhithich.uk [Wed Nov 09 20:18:22 2022] [error] [client 157.55.39.211:0] PHP: syntax error, unexpected '(' in /home/cluster-sites/2/f/fhithich.uk/public_html/php.ini on line 1
fhithich.uk [Wed Nov 09 20:18:32 2022] [error] [client 87.75.112.183:0] mmap() failed: [12] Cannot allocate memory
fhithich.uk [Wed Nov 09 20:18:32 2022] [error] [client 87.75.112.183:0]
fhithich.uk [Wed Nov 09 20:18:32 2022] [error] [client 87.75.112.183:0]
fhithich.uk [Wed Nov 09 20:18:32 2022] [error] [client 87.75.112.183:0] mmap() failed: [12] Cannot allocate memory
fhithich.uk [Wed Nov 09 20:18:35 2022] [error] [client 144.76.68.17:0] PHP: syntax error, unexpected '(' in /home/cluster-sites/2/f/fhithich.uk/public_html/php.ini on line 1
fhithich.uk [Wed Nov 09 20:18:36 2022] [error] [client 64.62.252.174:0] PHP: syntax error, unexpected '(' in /home/cluster-sites/2/f/fhithich.uk/public_html/php.ini on line 1
This is all pretty meaningless to me but seems to indicate something wrong on the php.ini file.
I did manage to find and have a look at this file and it is just the one line:
<?php phpinfo(); ?>
Any further help appreciated.
The php.ini file belongs to your hosting. This is used by some hosters in the form to set settings for PHP. The support of your hoster should be able to clarify this.