What kind of client do you have selected? Full node, light client or remote client?
You can check this on http://my.dappnode/#/system/repository
System > Repository
If you have chosen full node, maybe the get node is not synced yet, you can turn on the button that says “use remote during syncing or error” so the dappstore will be available while your get node is syncing
Update: The error messages are from a fresh install of DAppNode connected via Wifi. The problems are resolved, after I reinstall it connected by wired ethernet cable. Apparently, others are running into the same issues.