When you changed database collations, how did you do it? Did you export your data using the encoding you had and then convert the database dumps and then reimport? If not, you may have introduced encoding problems.
Some data types in OJS are stored as serialized strings of text, and PHP will unserialize these strings to convert them back into arrays. If the stored length of the string no longer matches the calculated length of the string, you will see errors like the ones you are posting. These errors can occur if the stored serialized string has a character that ends up consuming a different number of bytes in the new collation.
Best
Jason