Am having the same problem… I was fed up with the space taken by several checkboxes so replaced them with an array of checkboxes (content gets similarly serialized).
I got around it because in my case I knew it would always be six strokes added to the front and two to the back (check in your case*): Directly in the database (via phpmyadmin, mysql in my case) I told that column to strip those off… In detail,
My meta_key was (say) “marcel_data” and my database-prefix was (say) “wpdb_”, so I executed the following sql statement:
UPDATE wpdb_postmeta SET meta_value=SUBSTRING(meta_value, 6) WHERE meta_key = ‘marcel_data’
OK, for safety’s sake either first backup the table [easy: export as SQL file, just that table]. Or do it first without changing the data, like
SELECT id, meta_key, meta_value, SUBSTRING(meta_value, 6) FROM wpdb_postmeta WHERE meta_key = ‘marcel_data’
[come to think of it, if you’re not used to SQL –like me– just take the exported backup, open in an editor, and use GREP to find the pattern to strip — s:[0-9]+:” see, your “58” can now be “5” or “558” and it all works — then when satisfied drop the table and insert your edited version.]
Funny thing is that the grammatical error (at the end there’s 2 unwanted characters “;) gets ignored. If you ever edit the field in the admin then save, then the error corrects itself; if not, it’s there but things work. It’s neater if you correct the same, with a SUBSTRING from the end.
(*lenght to chop off: in your example, the “58” is the total number of keystrokes that follows, will it go beyond 99, to three digits? If so, you may need something smarter either in the WHERE-condition, or instead of SUBSTRING maybe LEN or so first to check)
