Again, you need to make a clone of your site and migrate the clone. Once you know what to expect you can manually fix whatever needs manually fixing. No migration is going to be 100% as we can't account for everyones different sites with different extensions/plugins and different server configurations. It's entirely possible for all or some of the migration to fail.

1) the whole configuration of the old CB was not taken over to the new CB. How can i fix this?

You will need to manually reconfigure or wait for this to be fixed if there is an issue found. I've added a bug ticket to look into this further.

forge.joomlapolis.com/issues/6083

2) the conditions of the auto actions were lost during the migration. I am not sure how to fix this.

I already have a bug ticket to investigate this further. You will need to manually reconfigure the auto action conditions or wait for this to be fixed if there is an issue found.

forge.joomlapolis.com/issues/6076

4) I cannot find the CB Paid Subscriptions Integration Plugin on the download page.

I've already provided the links for you. It is on the downloads page under CB Plugins for just the main plugin or you can download the Preset that includes CBSubs and all of its integrations. There is even a filter on the right side to filter to CBSubs downloads.

I just noticed that "Subscribers User access level settings" is indeed available in a non-merchanidse plan.

Merchandise plans can't manage access as they've no state to check against. Best I can suggest is try forcing it using CB Auto Actions and the below usage.

Plan Active
Global
Triggers: onCPayUserStateChange
User: Automatic
Access: Everybody
Conditions
1: [var3] Equal To PLAN_ID_HERE
2: [var2] Equal To A

Plan Expired
Global
Triggers: onCPayUserStateChange
User: Automatic
Access: Everybody
Conditions
1: [var3] Equal To PLAN_ID_HERE
2: [var2] Not Equal To A

Maybe there is a misunderstanding. You provided links, yes, but I cannot find what I am looking for.

Then I don't know what to tell you. Our downloads page as filters on the right and a search functionality. The CBSubs page it self below even has a download button that downloads the preset.

www.joomlapolis.com/addons/cb-paid-subscriptions

In CBSubs there was an Integration plugin tab called "Paid Subscriptions" and it was possible on a merchandise plan to activate another plan multiple times if the merchandise is purchased. Which integration plugin is doing this now? I cannot find the correct one under the links you provided.

There is no Paid Subscriptions CBSubs integration. I'm sorry, but I've no idea what plugin you're talking about. The plugin is "CB Paid Subscriptions" there is no "CBSubs Paid Subscriptions" and is maybe why you're having a hard time finding things.

If you need a plan to allow multiple purchases and it isn't a Merchandise or Donation plan (they both already allow multiple purchases) you can enable this with the "Multiple subscriptions per user" parameter under "Workflows" of your Subscription plan. A vast majority has not changed and certainly not from CBSubs 3.x to CBSubs 4.x. Please read your CBSubs documentation carefully.

This is odd, because it was possible with the Usergroups Integration under CBSubs 3.0.0. What a pity. But maybe we can work around this using Autoactions.

Assigning usergroups was done exactly the same way in CBSubs 3.0 as it is in CBSubs 4.0. I've reviewed the source and plugins available to CBSubs 3.0 and there is no CBSubs Usergroups plugin. I'm at a complete loss as to what plugin you're talking about. Maybe you're referring to CBSubs GroupJive? GroupJive has nothing to do with Usergroups, but forces users to join a CB GroupJive group.

Is there a way to install multiple CB plugins at the same time?

Yes, on our downloads page below select what you want to download using the checkboxes then click Download Package at the bottom of the page.

www.joomlapolis.com/downloads