USG Flex 500H - Problems with Services that require authentification

We have a new USG Flex 500H.

It is set up as firewall and gateway, connected via a known to be stable modem on a stable 1GBit/s fiber optic line.

With the new firewall we now have issues with some services like Outlook, Teams and Jira (Atlassian) where they randomly will no longer load/authentification will fail.

On the firewall we only use the default security policy control rules.

For debugging purposes I have turned of all security services. The issue persists.

In the log (debug) we see a lot of packages from Microsoft servers dropped with the message: "invalid state detected DROP". Connections are usually to ports above 5000 but a lot are also to port 443.