Sprout Forms + Email - internal server error Sprout Forms 2.5.1
Sprout Email 3.0.3

I am getting the error "Internal Server Error - Illegal string offset 'template'"

I have updated the template file to use email variable instead of entry for the html and text content respectively.

Stack trace: 

******************************************************************************************************
    2017/11/06 20:38:21 [error] [php] Illegal string offset 'template' (/home/forge/www.sitename.co.uk/craft/plugins/sproutemail/services/SproutEmailService.php:212)
    Stack trace:
    #0 /home/forge/www.sitename.co.uk/craft/plugins/sproutemail/services/SproutEmailService.php(69): Craft\SproutEmailService-&gt;error()
    #1 /home/forge/www.sitename.co.uk/craft/plugins/sproutemail/services/SproutEmailService.php(468): Craft\SproutEmailService-&gt;renderObjectTemplateSafely()
    #2 /home/forge/www.sitename.co.uk/craft/plugins/sproutemail/integrations/sproutemail/mailers/SproutEmail_DefaultMailer.php(99): Craft\SproutEmailService-&gt;renderEmailTemplates()
    #3 /home/forge/www.sitename.co.uk/craft/plugins/sproutemail/services/SproutEmail_NotificationEmailsService.php(487): Craft\SproutEmail_DefaultMailer-&gt;sendNotificationEmail()
    #4 /home/forge/www.sitename.co.uk/craft/plugins/sproutemail/services/SproutEmail_NotificationEmailsService.php(383): Craft\SproutEmail_NotificationEmailsService-&gt;relayNotificationThroughAssignedMailer()
    #5 /home/forge/www.sitename.co.uk/craft/plugins/sproutemail/services/SproutEmail_NotificationEmailsService.php(359): Craft\SproutEmail_NotificationEmailsService-&gt;handleDynamicEvent()
    #6 unknown(0): Craft\SproutEmail_NotificationEmailsService-&gt;Craft\{closure}()
    #7 /home/forge/www.sitename.co.uk/craft/plugins/sproutemail/services/SproutEmail_NotificationEmailsService.php(298): call_user_func_array()
    #8 /home/forge/www.sitename.co.uk/craft/app/framework/base/CComponent.php(567): Craft\SproutEmail_NotificationEmailsService-&gt;Craft\{closure}()
    #9 /home/forge/www.sitename.co.uk/craft/plugins/sproutforms/services/SproutFormsService.php(79): Craft\SproutFormsService-&gt;raiseEvent()
    #10 /home/forge/www.sitename.co.uk/craft/plugins/sproutforms/services/SproutForms_EntriesService.php(246): Craft\SproutFormsService-&gt;onSaveEntry()
    #11 /home/forge/www.sitename.co.uk/craft/plugins/sproutforms/services/SproutForms_EntriesService.php(195): Craft\SproutForms_EntriesService-&gt;callOnSaveEntryEvent()
    #12 /home/forge/www.sitename.co.uk/craft/plugins/sproutforms/controllers/SproutForms_EntriesController.php(162): Craft\SproutForms_EntriesService-&gt;saveEntry()
    #13 /home/forge/www.sitename.co.uk/craft/app/framework/web/actions/CInlineAction.php(49): Craft\SproutForms_EntriesController-&gt;actionSaveEntry()
    #14 /home/forge/www.sitename.co.uk/craft/app/framework/web/CController.php(308): CInlineAction-&gt;runWithParams()
    #15 /home/forge/www.sitename.co.uk/craft/app/framework/web/CController.php(286): Craft\SproutForms_EntriesController-&gt;runAction()
    #16 /home/forge/www.sitename.co.uk/craft/app/framework/web/CController.php(265): Craft\SproutForms_EntriesController-&gt;runActionWithFilters()
    #17 /home/forge/www.sitename.co.uk/craft/app/framework/web/CWebApplication.php(282): Craft\SproutForms_EntriesController-&gt;run()
    #18 /home/forge/www.sitename.co.uk/craft/app/etc/web/WebApp.php(823): Craft\WebApp-&gt;runController()
    #19 /home/forge/www.sitename.co.uk/craft/app/etc/web/WebApp.php(287): Craft\WebApp-&gt;_processActionRequest()
    #20 /home/forge/www.sitename.co.uk/craft/app/framework/base/CApplication.php(185): Craft\WebApp-&gt;processRequest()
    #21 /home/forge/www.sitename.co.uk/craft/app/index.php(62): Craft\WebApp-&gt;run()
    #22 /home/forge/www.sitename.co.uk/public/index.php(19): require_once()
    REQUEST_URI=/contact-us
Although the Sprout Email docs state it is preferable to use shorthand syntax for variables in notifications, I found this to not be the case for the Notification subject line.

So using {companyName} in the subject would cause the issue shown above.

{{object.companyName}} fixes this.