"WARN : Found UNKNOWN backup block: 0 at: **********" message is encountered within the application log files when investigating data source (4377124)

"WARN : Found UNKNOWN backup block: 0 at: **********" message is encountered within the application log files when investigating data source

"WARN  : Found UNKNOWN backup block: 0 at: **********" message is encountered within the application log files when investigating source data

In these type of situations, ApexSQL Log is unsuccessfully trying to read an unknown backup block, ie the one that cannot be labeled to other database backup or transaction log backups you're reading at present moment.
This process of trying to find out the source of the data is causing tool to slow down, hang or even unexpectedly crash.

The solution here would be to, when reading trn files, make sure you are investigating a full backup chain.
To do so, please consult following article for more details:
https://knowledgebase.apexsql.com/create-and-maintain-full-chain-of-transaction-log-backups/