If you run into the following error message when submittinga Soapbox Event registration form, it is the result of a known issue with an older version of a 3rd-party application called Rollup Helper that blocks the Campaign Member creation from the Lead object.
sbxd.sbx_DonationsCampaignMember : sbxd.sbx_DonationsCampaignMember: execution of AfterInsert caused by: System.DmlException: ConvertLead failed. First exception on row 0; first error: UNKNOWN_EXCEPTION, System.DmlException: Insert failed. First exception on row 0; first error: CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY, rh2.PS_Contact: execution of AfterInsert caused by: System.SObjectException: Field Contact.PhotoUrl is inaccessible in this context (rhx) :  (System Code) :  (sbxd)
Upgrade Rollup Helper packages to the latest versions from http://rh.passagetech.com and that should do the trick. If you are using the real-time rollup package, be sure to update that one as well.
Enjoy the additional rollup power in combination with Soapbox Engage!