OWA error: Exception type: Microsoft.Exchange.Data.Storage.StorageTransientException
From error message "Could not find any available Global Catalog in forest xxx.local", it indicate that this issue may related to the status of GC. Run DCDiag to analyzes the state of domain controllers.
Also, run below command to view the current DC, GC and static DC, GC:
Get-ExchangeServer -Status | fl Name,Current*,Static*