Dear Edwin,
we have installed the multisites on the Master website and on each of our slave sites

In the Master site on the MANAGE SITES Menu it shows for each slave site the following fields

| Id | Site Name | Domain :with and without www | DB Server :localhost | DB - different in each site xxxxx_xxxx | Prefix: joom_ |User: same as db server different for each | Status: Confirmed|

Our slave sites are not able to find the articles or the categories of the pages either in the master domain site or in the slave sites domains

we have already installed the extensions com_multisitescontent_1.3.01
and com_multisites_v1.3.29

what might be wrong and what might be the solution

First, JMS should only be installed in the master and is not required in the slave sites.
The only case where you may have to install JMS in a slave site is when you are using JMS full edition and want to propose a service for the website creation from the front-end.
Otherwise, the management of the slave site should only be performed from the master.

I don't know which version of Joomla you are using but it would be better that you reo the test with the latest JMS 1.3.33 version that perhaps fix that.

I have an issue with the menu item "Check for updates". Whenever I click "Check for updates" I receive

The CheckUpdates files received is not valid.
•The component is probably not registered.

Where can I reregister the component?

The problem is due to a bug introduced in recent Joomla that we reported to them.
See Bug reported in Github 5238
github.com/joomla/joomla-cms/pull/5238#issuecomment-64926131

While this is not fixed by Joomla, you need to perform the "Get Latest version" of the product in this website.
See the FAQ for the procedure to get the latest version.
/en/faq/faq-joomla-multi-sites#cat-122

The other alternative is that you fix the bug in the source of Joomla as explained in the GitHub.