Stderr is filemng failed: filemng: Error occurred during /usr/bin/base64 command.

Source: Backup set from Plesk 12.5.30, CentOS 7.1, 64-Bit
Destination: Plesk Onyx 17.5, CentOS 7.3, 64-Bit

Error on some, not all domains (only 1.3% of subscriptions, 0.46% of domains):
"Stderr is filemng failed: filemng: Error occurred during /usr/bin/base64 command."

After restoration it seems that all data is present. But what exactly is causing this error and what are the consequences?

Example log excerpt:

Stderr is filemng failed: filemng: Error occurred during /usr/bin/base64 command.