Getting an error when showing the bulletin templates (code 500)

I have been using the new version of mailpoet and it worked ok.

Suddenly I got this error while waiting to show the bulletin templates:

An error has happened while performing a request, the server has responded with response code 500

I read on the forum that it can be due to Jetpack plugin. And I deactivated it and the problem was solved.

I continued working and after a while… I got the same problem again:

An error has happened while performing a request, the server has responded with response code 500

I have no idea about what can have happened. I deactivated the plugin, deleted it and reinstalled it… but still the same problem.

This is the information of my website if that helps:

name: José Ramón
email: [email protected]
PHP version: 5.4.45
MailPoet Free version: 3.0.0-rc.2.0.2
MailPoet Premium version: N/A
WordPress version: 4.8.1
Database version: 5.5.55
Web server: Apache
Server OS: Linux archimadrid.com 2.6.32-696.1.1.el6.x86_64 #1 SMP Tue Apr 11 17:13:24 UTC 2017 x86_64
WP_MEMORY_LIMIT: 40M
WP_MAX_MEMORY_LIMIT: 256M
WP_DEBUG: false
PHP max_execution_time: 60
PHP memory_limit: 256M
PHP upload_max_filesize: 16M
PHP post_max_size: 8M
WordPress language: es_ES
Multisite environment?: No
Current Theme: Avada (version 5.2.2)
Active Plugin names: LayerSlider/layerslider.php, akismet/akismet.php, codestyling-localization/codestyling-localization.php, coming-soon/coming-soon.php, contact-form-7-to-database-extension/contact-form-7-db.php, contact-form-7/wp-contact-form-7.php, eventON/eventon.php, featured-video-plus/featured-video-plus.php, fusion-builder/fusion-builder.php, fusion-core/fusion-core.php, mailpoet/mailpoet.php, podpress/podpress.php, revslider/revslider.php, tinymce-advanced/tinymce-advanced.php, user-role-editor/user-role-editor.php, wp-db-backup/wp-db-backup.php, wp-members/wp-members.php, wysija-newsletters/index.php, nextgen-gallery/nggallery.php
Sending Method: PHPMail
Sending Frequency: 70 emails every 5 minutes
Task Scheduler method: WordPress
Cron ping URL: https://www.cursillosmadrid.es?mailpoet_router&endpoint=cron_daemon&action=ping
Default FROM address: [email protected]
Default Reply-To address: [email protected]
Bounce Email Address: [email protected]
Total number of subscribers: 1800