I had this issue, and saving the theme options did not work for me.
Using Google developer tools, I saw that our problem was the plugin trying to load the css file using the server path to the theme (not the URL). For my site that was returning https://[SERVER-IP]/%5BDOMAINNAME.com%5D/wp-content/plugins/all-in-one-event-calendar/public/themes-ai1ec/vortex/css/ai1ec_parsed_css.css
Instead of https://[DOMAINNAME.com]/wp-content/plugins/all-in-one-event-calendar/public/themes-ai1ec/vortex/css/ai1ec_parsed_css.css
In case it helps anyone else, I was able to fix it by editing the code around line 170 of
all-in-one-event-calendar/lib/css/frontend.php
I replaced
return Ai1ec_Http_Response_Helper::remove_protocols( apply_filters( ‘ai1ec_frontend_standard_css_url’, $theme[‘theme_url’] . ‘/css/ai1ec_parsed_css.css’ ) );
with
$themeurl=str_replace(‘[SERVER-IP]/[DOMAINNAME.com]’,'[DOMAINNAME.com]’,$theme[‘theme_url’]);
return Ai1ec_Http_Response_Helper::remove_protocols(
apply_filters(
‘ai1ec_frontend_standard_css_url’,
$themeurl . ‘/css/ai1ec_parsed_css.css’
)
);
That has fixed the styling for me.