Hi there. Unfortunately I don't have an Odoo server handy just at the minute and from the Odoo appliance build code I can't see explicitly what the default Odoo config is.

On face value, it appears that either you are running in "multi-threaded" mode (i.e. 'workers=0') - which according to the error message is not supported if you wish to render PDFs. Another possibility is that you are running in "multi-process" mode (i.e. 'workers>=1') and either only have 1 worker defined, or have multiple workers defined but for some reason they are being killed or crashing (e.g. timing out due lack of CPU resources, or running out of RAM).

It's also possible that the error message is a complete red herring and something else entirely is going on...

First thing I suggest that you do is check how many workers you have configured. IIRC the config file should be /etc/odoo/odoo.conf. As a general rule, Odoo recommend setting 'workers=(cpu-threads x 2) +1' e.g. if you have 2 threads/cores; 'workers=3'. If you make any adjustments there, you'll need to restart Odoo to apply them. I.e. this should do the trick:

service odoo restart

FWIW her's the relevant section from the Odoo docs:

Worker number calculation
Rule of thumb : (#CPU * 2) + 1
Cron workers need CPU
1 worker ~= 6 concurrent users
memory size calculation
We consider 20% of the requests are heavy requests, while 80% are simpler ones
A heavy worker, when all computed field are well designed, SQL requests are well designed, … is estimated to consume around 1Go of RAM
A lighter worker, in the same scenario, is estimated to consume around 150MB of RAM
Needed RAM = #worker * ( (light_worker_ratio * light_worker_ram_estimation) + (heavy_worker_ratio * heavy_worker_ram_estimation) )

If that doesn't help and/or you already have multiple workers configured, then the next step is to check the logs to see if you can find more info about exactly what is going wrong (e.g. worker processes crashing due to insufficient memory). I'm not 100% sure, but I suspect that you should be able to find the Odoo log file in the normal place for logs (i.e. /var/log). If you're having problems interpreting it, please feel free to post the last few lines into a new post (or attach a separate file to your original post - only the first post in each thread can have an attachment).

If you get stuck or my instructions don't seem to assist, please let me know what/how/where and when I get time I can launch an Odoo server and try to see if I can reproduce the issue.

I think this is unlikely (as I would have expected others to complain sooner), but on other possibility that has just occurred to me is that perhaps the version of wkhtmltopdf that we preinstall from Debian repos) is sub-optimal when used with Odoo?

I also would have expected it to give a different error, or perhaps even a blank pdf if that was the issue.

The only reason that I note it is that I recall that we had issues with that back in the v14.x release of the Odoo appliance (which included Odoo 8). Although I was pretty sure that it was resolved for TurnKey v15.x...