It maybe a setting in your browser doing it. I am using Chrome and it works fine here for your own site. Review your browser security settings carefully.

I need your opinion about this, could this be the issue? What value would you recommend to set session.cookie_lifetime to?

If cookie_lifetime is set to 0 then cookie lifetime is as long as the browser session. This means as long as the browser window isn't closed the cookie is still active. Sounds like you've a server issue though if that's not working properly.

What exactly do you mean with "unreliable hosts"?

A host with poorly configured or maintained servers and little or no support. Could be as simple as poorly configured php.ini file though. There's more bad hosts than there is good. I typically stick with hosts like SiteGround, A2Hosting, or I purchase a VPS like Vultr and self host (preferred as I've 100% control over the server).

In short absolutely stay away from Endurance International Group (EIG) hosting. This includes hosts like Bluehost, HostGator, etc.. They own upwards of 60+ hosting providers. They're all terrible. They own hundreds of different reseller domains as well. GoDaddy isn't EIG, but it's just as bad.

I have the same problem when the joomla cache plugin is enabled.

That's normal and expected. Joomla page cache caches the page buffer. This means it caches the security token used for form validation as well. The tokens are single use per form session. So it results in the cache being out of date. Do not use Joomla page caching on anything but static websites; it just doesn't make sense to use it otherwise and does not work with any form of dynamic environment. Its entire purpose is to fulfill blog site needs. Just enabling Joomla cache won't cause this; it's specifically the Page Cache plugin.

Spiralmind wrote: My site was doing this on SiteGround (reliable) and the solution seemed to be disabling their System - jSGCache plugin

I realize many things are best left uncached but I may try speed cache instead. Then I can turn off CB pages. SiteGound is all or nothing I think.

That's not really a hosting issue so much as it is a caching extension issue. SiteGround just happens to provide such an extension, but it has nothing to do with their server configuration.

You are unlikely to see significant performance gains from caching the entire output buffer, which is what that caching extension and Joomlas page caching does. You are gaining performance from the headers being minimized, which a full page cache does in addition to caching the HTML (which is pointless beyond high traffic static pages). This means JS and CSS files processed into 1-2 files vs say 20. This reduces the header load of your site significantly. For example on Joomlapolis we gain a 60% page load performance boost by just caching and combining JS/CSS. This is further boosted by lazy loading images. All of which is done with JCH Optimizer (tested and confirmed compatible with CB) and at this time is the only cache extension I can really recommend (we have not tested any others).