Problem determining which core cron job is failing

[stevenospam]

Support Request Template
What version of phpBB are you using? phpBB 3.3.3
What is your board's URL? http://forum.altosdecerroazulresidencial.org
Who do you host your board with? Lonex
How did you install your board? I used the download package from phpBB.com
What is the most recent action performed on your board? Fresh Install
Is registration required to reproduce this issue? Yes
Do you have any MODs installed? No
Do you have any extensions installed? Yes
What extensions do you have installed? Digests
What styles do you currently have installed? prosilver
What language(s) is your board currently using? english & spanish
Which database type/version are you using? MySQL 5
What is your level of experience? New to PHP and phpBB
What username can be used to view this issue? No answer given
What password can be used to view this issue? No answer given
What actions did you take (updating your board; installing a MOD, style or extension; etc.) prior to this problem becoming noticeable? I downloaded the install package and used it to update my existing 3.0 phpbb installation. There were no problems with the update process.

I have a system cron setup as follows:
curl -s -S https://forum.altosdecerroazulresidenci ... .cron_task

After the cron runs, the output that gets emailed to me is: GIF89a ? ÿÿÿ !ù , D ;

PHPBB3_config cron_lock is left non zero like 1620072012 8xt0ttnbrisxysjt

cron:list produces:
Tarea preparada:
================

* cron.task.core.prune_all_forums
* cron.task.core.prune_notifications
* cron.task.core.tidy_cache
* cron.task.core.tidy_plupload
* cron.task.core.tidy_search
* cron.task.core.tidy_sessions
* cron.task.core.tidy_warnings
* cron.task.text_reparser.post_text
* cron.task.text_reparser.user_signature
* phpbbservices.digests.cron.task.cron_task

Tarea no preparada:
===================

* cron.task.core.prune_forum
* cron.task.core.prune_shadow_topics
* cron.task.core.queue
* cron.task.core.tidy_database
* cron.task.text_reparser.pm_text

I need to determine which of the core cron tasts is failing and then I need to determine why it's failing. I am comfortable modifying php files but I don't know the language.

Your help is appreciated!