AFAIK you need no conversion. I can open db's older than 2010 no problem. You don't say what bit version of Office and Windows that Access 365 is running on. If 64 you could have an issue there, but I've never seen that error message due to bitness issues. If not 64 bit then disable any startup code and default opening form (I presume you can shift bypass). Then check for broken/missing references as a start. If none found, try compiling the code and fix any errors that may arise. If still no luck, put a break on the startup code and step through until you raise an error. Just saying it errors on a Shell command line doesn't help us much. We'd need to know the error message and number, and see the code.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.