SQL Agent fails to backup to network automatically but works manually

Hi,

I have multiple SQL servers all configured to backup in to a single network folder on our local NAS which then syncs to Dropbox.

I've created a maintenance plan in each SQL server and when i test it manually by right clicking the Maintenance plan run clicking on Execute, it runs successfully but during schedule runs it always fails for no apparent reason.

Can you help me figure out the problem

below is the T-SQL log

BACKUPDATABASE[SandsV8Demo]TODISK=N''\\10.10.20.100\dropbox\HostingDBs\SandsV8Demo_backup_2019_08_08_093001_8952765.bak''WITHNOFORMAT,NOINIT,NAME=N''SandsV8Demo_backup_2019_08_08_093001_8952765'',SKIP,REWIND,NOUNLOAD,COMPRESSION,STATS=10GOdeclare@backupSetIdasintselect@backupSetId=positionfrommsdb..backupsetwheredatabase_name=N''SandsV8Demo''andbackup_set_id=(selectmax(backup_set_id)...