[Can't upload non-standard sized banners]

Further information: it is not limited to non-standard sizes, has been doing it consistently with a 300x250 banner. Note that I am able to upload a banner (very) intermittently.

This is what I see in the CPanel Error log:

[Mon Jan 02 21:30:23 2017] [error] [client 65.191.5.155] File does not exist: /home/nnnnnn/public_html/408.shtml, referer: http://mysite.com/adserver/www/admin/banner-edit.php?clientid=4&campaignid=4
 

Any thoughts are appreciated.

[Can't upload non-standard sized banners]

I can upload IAB-standard-sized banners with no issue, but I get a CONNECTION_RESET browser error if I try to upload a custom-sized banner image. All graphics used are PNG format.

Am I missing something?

 

Thanks!