Plesk interface on Plesk Onyx 17.5 and earlier is not accessible: syntax error, unexpected ':', expecting ';' or '{' (WelcomePanelTrait.php:15)

Symptoms

Plesk interface on servers with Plesk Onyx 17.5 and earlier is not accessible:

ERROR: syntax error, unexpected ':', expecting ';' or '{' (WelcomePanelTrait.php:15)

Cause

Bug EXTPLESK-5618.

Resolution

The bug is already fixed, the issue will be resolved after the next execution of the Plesk daily task.
The execution of the Plesk daily task for resolving the issue can be forced as follows:

1. Connect to the server using SSH.

2. Execute the command:

   # plesk daily UpgradeExtensions