Module update fails on #7004

Rephrasing the issue title.

Update attempt with Drush on Beta4:

The following updates are pending:

metatag module : 
  7004 -   Update all language values in the metatag table, will also resolve problems  created during the release of beta3. 

Do you wish to run all pending updates? (y/n): y
Performed update: metatag_update_7004                                                                                                                                    [ok]
Performed update: metatag_update_7004                                                                                                                                    [ok]
Performed update: metatag_update_7004                                                                                                                                    [ok]
Performed update: metatag_update_7004                                                                                                                                    [ok]
Performed update: metatag_update_7004                                                                                                                                    [ok]

Fatal error: Class name must be a valid object or a string in /[path-to-dev]/html/includes/common.inc on line 7749
Drush command terminated abnormally due to an unrecoverable error.                                                                                                       [error]
Error: Class name must be a valid object or a string in /[path-to-dev]/html/includes/common.inc, line 7749
The external command could not be executed due to an application error.                                                                                                  [error]

I downloaded latest Metatag -dev release. Still failure:

The following updates are pending:

metatag module : 
  7004 -   Update all records in the metatag table and assign each the correct language  value, will also resolve problems created during the release of beta3. This  might take a while, depending on how much data needs to be converted. 
  7005 -   Removing wrong metatag watchdog entries that break the adminreportsdblog  page. 
  7006 -   Remove metatag records that were added by old versions of the module for  entities that don't actually support meta tags. A more complete version of  this will be added later on after it's (hopefully) guaranteed that all  modules have updated to the correct API usage. 
  7007 -   Remove metatag records for objects that have been deleted; older versions of  Metatag may have failed to purge these. 
  7008 -   Remove metatag any empty records that may be hanging around from old  releases. 

Do you wish to run all pending updates? (y/n): y

Fatal error: Class name must be a valid object or a string in /[path-to-dev]/html/includes/common.inc on line 7749
Drush command terminated abnormally due to an unrecoverable error.                                                                                                       [error]
Error: Class name must be a valid object or a string in /[path-to-dev]/html/includes/common.inc, line 7749
The external command could not be executed due to an application error.                                                                                                  [error]
Finished performing updates.                 

Running update.php yields a drupal_set_message() error:

An AJAX HTTP error occurred. HTTP Result Code: 200 Debugging information follows. Path: http://example.com/update.php?op=selection&token=_4sKC2pjKGoq73HNrdH1NK6OT083ZN75renuu-o8r_Q&id=230&op=do StatusText: OK ResponseText: Fatal error: Class name must be a valid object or a string in /[path-to-dev]/html/includes/common.inc on line 7749

There's a prompt to take me to the error page, but then I'm taken to a success screen:

Update #7004

    50 records were updated in total.

However, if I go back to update either by update.php or drush up, update 7004 is still prompted as necessary.

Last bit of info: Watchdog entries:

• Update 7004: 66 records to upgrade.
• Update 7004: 50 records were updated.
• Update 7004: 50 records were updated in total.

And yet repeating the process generates the identical messages. The counts do not change.

I hope this info is helpful.

I am seeing the same error prompt as described above.

Is it possible to skip an update from the UI without using drush ? I can update any other module now. I was thinking to uninstall Meta tags and reinstall it unfortunately there are to many nodes having custom descriptions and i can't find the table where theses descriptions are stored so that i can copy paste them.

Ok I was able to install updates from other modules by disabling metatags and deleting the module folder.
Now I would like to resolve that bug and I am willing to completely uninstall Meta tags and recreate all my custom descriptions if needed but i need a way to list all pages that use one and save them in a way. The metatag table in the database is not clear for me. I was thinking to do it with views but the metatag fields are not available... How can I make a summary of all my custom descriptions and titles before I erase everything ?

Thank you

Noting that it's becoming apparent that this may be the same issue as #1844638: metatag_update_7004 goes into an infinite loop. There's a patch pending there. (I can't test it anymore. I uninstalled and reinstalled Metatag with no problem, and moved on.)

This is ultimately a duplicate of #1844638: metatag_update_7004 goes into an infinite loop, lets focus on that issue.