Theme won't change from default

I have successfully installed and used several theme templates through FTP, however now I can no longer select a theme other than the default. When I select a different theme and check the tick box, the newsletter saves but does not use the selected theme. I disabled all plugins and it still does not work. The message I am getting in WYSIJA QUERIES is the following:

0
array
duration 0.00047612190246582
query SELECT email_id FROM wp_uerb_wysija_email WHERE email_id=101
1
array
duration 0.00065088272094727
query SELECT * FROM wp_uerb_wysija_email WHERE email_id=101 ORDER BY email_id DESC LIMIT 0 , 1
2
array
duration 0.00071978569030762
query SELECT * FROM wp_uerb_wysija_user WHERE wpuser_id=1 ORDER BY user_id DESC LIMIT 0 , 1

All plugins and themes are updated to latest versions. I would appreciate any insight.

https://www.ads-software.com/plugins/wysija-newsletters/