SQL Server Availability Group LeaseTimeout and slow IO
how can slow IO-requests cause a lease-timeout? Can to many pending IO-requests cause a lease-timeout?

No, slow I/O requests can't cause a lease timeout directly.
However, if the server is completely overloaded (CPU at 100%), than can cause pending I/O requests and lease timeouts.  The default lease timeout is 20 seconds, and your pending I/O is 26 seconds.  High CPU or some other server / OS level problem is more likely the issue here.
Another cause is that SQL Server encountered a serious error, and is generating dump files (which causes the process to pause, potentially long enough for WSFC to think the lease timed out).
See the documentation for a couple more possibilities:

OS not responding, low virtual memory, working set paging, generating dump, pegged CPU, WSFC down (loss of quorum)

You should review the SQL Server error log to see if there are dumps being created.  If you have monitoring from the time of these incidents, you could also check for maxed out CPU.