I am not able to reproduce all the time.
It might be due to other side effect.
Something appears to be locked twice and it generate an error 500.
Moreover, when the wizard receives an expection of this type, the error popup is shown but the page is already created and the wizard still visible, so you can validate the creation again and it will create a new page and content.

When trying to reproduce, I once get the following pattern:

• I create the page and content and choose to tag the content via the wizard
• The errors happened 3 times before I was able to create the page without error (I was able to validate the same wizard 3 times, see above paragraph)
• Then I create a new page from the start and get the error 2 times.
• Then only one time.
• Then I was able to create several pages without errors.

Once I do not have the error, it seems to works well for the several next page creations. I do not know how, but some time after, the error can arise again.