I encountered this issue the morning and after reading some of the current threads decided to perform a clean 2.5.5 install (new db) - then added & installed all add ons, templates, images, etc.. Once this was done I replaced the db name in the expressionengine>config>database.php to the old database and logged in. Thus far without any casualties…..
I have a suspicion that the issue is/was caused by the database, albeit a novice I noticed that it (still) contains tables of previously installed add-ons which have been superseded or after a short trial were rendered obsolete.
Could this have been the case and what is the best practice for maintaining/cleaning a database (even for a mysql neophyte)?