When Enforce is trying to index remote share, it's mounting it first via UNC path, however if drive letter is already used by another partition in the system, it will fail to map with an error that remote path could not be read. After restarting indexing, new letter is being used for mapping and indexing goes through successfully.
The path could not be read.
Data Loss Prevention (DLP) Enforce 15.5 MP1
DLP config has hardcoded list of letters to use for mapping drives and if that letter is used already on the system, mapping of remote path will fails.
To avoid this problem, edit the Crawler.properties file and remove the letters that are already in use by the system, hence it will not try to use those letters and you will avoid having this issue altogether.
File location: INSTALL DIR\Program Files\Symantec\DataLossPrevention\EnforceServer\15.5\Protect\config\Crawler.properties
# All valid drive letters to be used for mounting on windows
filesystemcrawler.mount.drive.validletters.windows = l:,m:,n:,o:,p:,q:,r:,s:,t:,u:,v:,w:,x:,y:,z: