well this is strange, we haven't heard this from anyone else upgrading...

besides that, does it work correctly?

The short answer is that it does not work correctly. Here is the long answer.

Oh and that may not be the only issue, but it is the only error message it displayed.

IM has trouble adding, updating and deleting Items in an invoice or quote. I checked the SQL after several failed attempts to add an item. The SQL #####_invoices_items had multiple duplicate records or partially complete records assigned to the same invoice_id as the invoice or quote I was attempting to edit. The name, desc, fields were often empty or partially complete. The value, amount and discount fields showed "0.0000", "0.00" and "0.0000" respectively for all of the records.

#####_invoices_items was not the only SQL table affected. #####_invoices_payments and #####_invoices_templateitems had similar issues.

After playing around with it I found something interesting. I noticed when adding items to an invoice or quote when the "Invoice Saved!" messages would appear the data entered in the new item fields would be removed. All the fields would be returned to the default placeholders. If the "Save" button was clicked the same thing would occur. However, the SQL record for that item would show the data that was entered into the fields at the time the item was saved. Any data entered after the first save would not be added to the SQL record for that item. Thus it is not possible to edit invoice items. The data entered at the point of the first item save was the permanent data in the SQL.

I installed IM 2.1.0 a second time. The second time it did not display the error message, however. all of the aforementioned issues persisted.

I did not play around with things much more than this.

if you can send me access at support@ joomlathat.com I'll be happy to take a look at it...

It was the cache. I suspected that may be causing it, but mistakenly believed I had disabled that for the back-end. Thank You.