Email migration from SBS 2011/Exchange 2010, to Office 365

I performed a migration from Exchange 2010 on SBS 2011 to Office 365. I'm preparing to turn off the old server and use a new DC, but for now the old server with AD and Exchange is still operational.

Everything has gone smoothly so far, and users have Office 365 directing mail to their computers. Great! However, some users are getting prompted for Exchange credentials. It turns out I need to convert their mailboxes in Exchange 2010 to MEUs to keep this from happening.

I followed all of the instructions here: https://community.office365.com/en-us/w/exchange/845.convert-exchange-2007-mailboxes-to-mail-enabled-users-after-a-staged-exchange-migration

Everything seemed on track, but then I get the following error for every mailbox:

An Active Directory error 0x51 occurred when trying to check the suitability of
 server 'atlanticserver2.atlanticthread.com'. Error: 'Active directory response
: The LDAP server is unavailable.'
    + CategoryInfo          : NotSpecified: (:) [Set-MailUser], SuitabilityDir
   ectoryException
    + FullyQualifiedErrorId : FF5E0243,Microsoft.Exchange.Management.Recipient
   Tasks.SetMailUser

Exception calling "InvokeSet" with "2" argument(s): "The server is not operatio
nal.
"
At C:\Exchange2007MBtoMEU.ps1:76 char:33
+         [Void]$ADUser.psbase.invokeset <<<< ('msExchMailboxGUID',$MailboxGUID
.ToByteArray())
    + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException
    + FullyQualifiedErrorId : DotNetMethodException

I've double and triple checked LDAP to make sure it's enabled and working. I've made sure my account has all of the necessary permissions, and my powershell console is being run as an administrator. I've googled my brains out, but apparently this error never happened to anyone else in the history of Office 365. I even contacted Office 365 support, but they claimed this wasn't covered by them, and that I'd need to pay for server support.

I'm hoping somebody else has seen this error, knows what causes it, and how to fix it. If I can't find a solution, it looks like the only option would be to throw the old server out the window, create a new domain, and reimage the new server.
SSsourceAsked:
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Vasil Michev (MVP)Commented:
If the end goal is to avoid the credential prompts, which are in turn caused by Outlook hitting the local Autodiscover, you can easily solve it by using GPO/reg keys: http://support.microsoft.com/kb/2212902

All you need for EO mailbox to be accessible is the HTTP redirect endpoint, you can block all others and the prompts should disappear.

As for the actual script, try running it step by step. What it does is basically to disable the mailbox and then enable mail-user with the same attribute values. The error might be due to old version of PowerShell for example.
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SSsourceAuthor Commented:
Hi Vasil,

I had already done the registry change. The issue isn't auto-discover, that was already changed over the other day. The issue is that they are logged into their computers with AD accounts that are still tied to Exchange mailboxes...so Outlook thinks it needs Exchange credentials even though it doesn't. Does that make sense?

I also ran the script step by step, and it gathers the user data from Office365, creates the cloud.csv file with the user data, then tries to use LDAP to access the users' info from AD/Exchange. That's where it is failing.

Right now I'm trying to disable Exchange on a test user's account to see if that breaks the association. Still waiting for user's permission though, so I'm hoping to get some answers in the meantime.
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SSsourceAuthor Commented:
I'm pretty confident now that I've got the solution.

Rather than running that script and fixing LDAP and such, all I had to do was right-click on the Exchange mailbox and select Disable. This disabled the Exchange mailbox while leaving AD alone. I'm not sure if I need the users to be mail contacts, if it will break something if I don't, so I just manually created the contacts.

I'm really baffled that this solution wasn't presented anywhere else online...Microsoft seemed very adamant that the only way to complete this migration was to run that script, and there was no way to manually convert Exchange mailboxes.
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SSsourceAuthor Commented:
I fixed the issue myself because the answer didn't exist anywhere on the internet. I wanted to document the answer in case anybody else has the same problem.
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