Can't back up Plesk with Acronis Backup: version `GLIBCXX_3.4.26' not found

Plesk for Linux kb: technical ext: acronis

Applicable to:

After updating Plesk Obsidian to version 18.0.40, Acronis backup fails with the following error:

Failed to execute command '/usr/lib/Acronis/BackupAndRecovery/webcpprecapture: /usr/bin/sw-engine: /usr/lib/Acronis/system_libs/8.2.1/libstdc++.so.6: version `GLIBCXX_3.4.26' not found (required by /usr/bin/sw-engine)
Acronis console shows the following error message:

Cannot protect a device with assigned quota
Protection plan 'webcp' has been revoked from 'server.example.com' due to a licensing issue.

A bug in the Acronis agent that has been fixed in version 15.0 28610.

Update the Acronis Agent (Recommended)

Log in to Plesk
Open Acronis Backup > Go to backup console (it may ask for credentials)
Go to Settings > Agents in the Acronis Backup console
Select the machines with an outdated agent. They're marked with an orange exclamation mark.

Note: The machines must be online.
Click Update agent

Manual Workaround

Connect to the Plesk server over SSH
Edit /usr/lib/Acronis/BackupAndRecovery/webcpprecapture

# vi /usr/lib/Acronis/BackupAndRecovery/webcpprecapture
Add the following lines:

unset LD_LIBRARY_PATH
unset LD_PRELOAD
Save the changes

If the Acronis agent is on version 28323 (can be checked in Acronis console), follow these steps too:

Connect to the Plesk server over SSH
Download the patched libwebcp_agent.so

# wget https://plesk.zendesk.com/hc/article_attachments/12376927359895/libwebcp_agent.so
Rename the original file:

# mv /usr/lib/Acronis/BackupAndRecovery/libwebcp_agent.so{,.backup}
Move the downloaded file to the Acronis directory:

# mv libwebcp_agent.so /usr/lib/Acronis/BackupAndRecovery
Restart the Acronis service:

# service acronis_mms restart
Verify that Plesk is detected:

Note: It make take a couple of minutes to take effect.

# acropsh -m dmldump -s mms -vs InstanceManagement::Instance | grep InstalledApplication

('.Parameters.InstalledApplication', 'item_string', 'Plesk'),
('.Parameters.InstalledApplication^Searchable', 'nil', None),