The 5250 protocol includes the ability for the host applications and system to save and restore complete screens. In the process of restoring a screen from the host, the restore buffer has too many fields compared to the active fields defined for the buffer.
This may indicate an interpretation error in the Inventu Viewer server, a need for an enhancement or a host programming error.
If a user can identify how this occurred you can use Selective Trace, otherwise try to reproduce using Global Tracing during a light usage period, then submit a ticket with the trace files to Inventu Support.