Of course (will be clear) no v2 plugins. Most likely it are plugins that do store a lot of data or will be capable of changing internal settings. (just as an example : take a third party clan plugin, those do often create connections with login and user functions.. old vs new code can stumble upon as they may ask for the same but can call the wrong settings (which might be changed). Does that make all elder plugins 'low quality'? No, there are some good old ones that work without a hassle.(and no, not every clan plugin is bad, just as example for 'runnings')
Due to the nature of php versions in use today, some could just throw in plain error mistakes.
I would suggest (manual labour helas): make a backup of your dbase. Use a FTP client and just transfer (v1) plugins OUT of the server folders, so it is not to be 'seen' by the system (aka delete from server, but keep on pc). do it 1 by one, and do not change anything else with them (do not change names as it will not eliminate the files inside if faulty). Each time (you can stay in your admin pref screen (logged in) refresh the prefs page, make a small insignficant change and try to save. Iff it works and get success, reverse the process...that way you can find out which plugin (or files within) does it.
If i can recall for me that time it was the third plugin ( yes a self made one uhhh with errors in code file) that did it (so was not to lenghty of a process.
When in doubt just repeat. Why the copy of dbase ?... v2 CAN (caching) sometimes alter some dbase settings in present/gone plugins... Iff things would alter too much . files are put in place on the fly, and having an untouched dbase ... you will understand..
Else i would not have another solution... sometimes it has to be done, and please note : the more older plugins in use : eventually (php 5.6 and higher) will bring in troubles on future use.. try to lookout iff there are newer ones already available which can do the (near) same. Hardest things will be the communication on MySQL (deprecated) > PDO is more and more in use...
Success and ask if needed. When success a notice would be fine too.. (solution which one or so would help)..