repeated backup failures

i have a very simple WP website. no ecommerce or tons of media. its a simple informational only website that has what i would consider low traffic. i have repeatedly tried to do a manual backup and it fails each time stating ‘The backup attempt has finished, apparently unsuccessfully’. and in the list of backups, i get plugins, themes, uploads but it also says (not complete). i have lowered the file size from 400mb to 300mb and 200mb with no success. i have other websites with the same type of server (different accounts) and have no problems. in the log file prior to completion is says

UpdraftPlus WordPress backup plugin (https://updraftplus.com): 1.16.6 WP: 5.0.3 PHP: 7.2.6 (litespeed, Linux a2plcpnl0624.prod.iad2.secureserver.net 2.6.32-896.16.1.lve1.4.54.el6.x86_64 #1 SMP Wed May 2 07:43:19 EDT 2018 x86_64) MySQL: 5.6.39-cll-lve WPLANG: en_US Server: Apache safe_mode: 0 max_execution_time: 900 memory_limit: 256M (used: 39.2M | 40.3M) multisite: N openssl: OpenSSL 1.0.2k-fips  26 Jan 2017 mcrypt: N LANG:  ZipArchive::addFile: N'
and '[Updraftplus] The backup is being aborted for a repeated failure to progress.
An error condition has occurred for the first time during this job.

any help? thanks!