After upgrading to latest UBUNTU version 18.4.1, on every machine i try using BOINC-manager to open the window for "options" and later "computing preferences" required to set max CPU load (to avoid overheating of quad core CPU), the windows opens but it is just WHITE: no more options visible to set CPU load limit.
The faulty ubuntu is the last version available in Italy.
Is there a command-line program i could use to set it? Is there any config file i could manually change by editing it? Thank you

For me, the best way to do this is to limit the number of cpu's rather than percentage of cpu time. Pretty sure there is an option in cc_config.xml to do this. Despite the xml extension of this file, edit in a text editor and save as a plain text file.

The option you are looking for is I think
<ncpus>N</ncpus>
Act as if there were N CPUs; e.g. to simulate 2 CPUs on a machine that has only 1. To use the number of available CPUs, set the value to -1 (was 0 which in newer clients really means zero).

So if you set it to three it will be the same as restricting BOINC to 75% of cpu's using BOINC Manager.

If I have got this wrong someone will be sure to give the correct version shortly.

When every screen is white, it means that you only started the manager, not the client. Under Linux starting BOINC Manager won't start the client, you have to start it (boinc-client) separately and prior to the manager.

When the computing options screen is white, it would seem that BOINC Manager - or part thereof - is missing something to render it completely. What you can try is to do an <code>ls boinc-manager</code> from a terminal window. That's El Es. If you can't read it all, copy & paste it here and a Linux guru will probably be able to tell you if something's missing and needed.

Don't use the cc_config.xml file to set the amount of CPU cores. That option is used for testing, and is primarily for running work on more cores than you have. So you can for instance set it to 64 on a 4 core CPU and run 64 tasks (slowly, because the 4 cores will run all 64 at the same time, 16 per core)

If you want to hand edit any file, edit global_prefs_override.xml, and then the max_ncpus_pct value. Do know it's percentages that are used here.

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If you want to hand edit any file, edit global_prefs_override.xml, and then the max_ncpus_pct value. Do know it's percentages that are used here.

That's what setting preferences via the BOINC Manager interface does: it creates an override file (which is optional and may not be present on your machine), and thus detaches you from the alternative method of setting preferences via a project web site. Check the Event Log after startup to see whether the line

18-Sep-2018 17:42:36 [---] Reading preferences override file

is present. If it is, edit it: if not, edit your global preference set on a project web site.

As Dave says, it's usually better to change the first preference ("Use at most nnn% of the CPUs"), rather than fiddling with CPU time.