Class does not exist. 'X' is the default value

What causes this issue?

Inconsistency in table TSL1D.
For each entry in table TSL1T (system log message texts) there must be
a corresponding entry in table TSL1D (problem class). The entry in
table TSL1D is missing. This error can only occur if transaction SE92
is by-passed.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Check the problem class, change it if necessary, then save it. This
will correct the inconsistency.

Error message extract from SAP system. Copyright SAP SE.