Exchange 2010 to Exchange Online Public Folder Migration Failed

Support got back to me and said this error can happen if the Public Folder name and the Display Name do not match. To check the names in PowerShell: Get-Mailbox -PublicFolder Mailbox1 | fl name,displayname

If mismatched (like mine are), I wrote the following PowerShell script to rename all of the public folder mailboxes to match the display name. If you only have a couple of mailboxes you could also just rename them in the EAC.

# Connect to EXO
$Credential = Get-Credential
Connect-ExchangeOnline -Credential $credential -ShowProgress $true

# Get all public folders
$PFM = Get-Mailbox -PublicFolder | Select-Object Name,DisplayName

# Loop through each public folder
ForEach ($M in $PFM){
# Define the mailbox display name
$MBDN = $M.DisplayName
# Define the mailbox name
$MBN = $M.Name
# Compare the name vs. display name
If ($MBN -ne $MBDN){
Write-Host "Name mismatch! Updating $MBDN now." -ForegroundColor Yellow
# Rename the mailbox to match the display name
Set-Mailbox -PublicFolder $MBDN -Name $MBDN
}
ElseIf ($MBN -eq $MBDN){ Write-Host "$MBDN is set correctly." -ForegroundColor Green}

}

I'm sure there is better/more efficient code but that worked for me. After the mailboxes have been renamed, try resuming the failed mailboxes and see if they start syncing as expected. In my case (still monitoring it) they at least did not fail right away so I believe it's working but only time will tell.