This is looks like the bug #8743409. However, this bug is fixed in 22.214.171.124. I cannot tell for sure if it is safe to ignore the error without knowing the root cause. However, if you are 100% sure that all affected tables are empty, you can exclude all their columns from conversion using the corresponding property on Column Properties tab. This should prevent DMU from complaining at conversion time. However, CLOB columns cannot be excluded from conversion, so you will have to either fix the actual problem or temporarily drop the affected tables with CLOB columns.
I appreciate your response to my query.
I checked, and this database is 126.96.36.199 (not 188.8.131.52 as I had posted). Is there any detail on this bug? I have searched but the only match I get is your response to my question.
These tables are empty, but when I turned off the scanning of the column as you suggested, I still got an error on the DMU scan. Can I proceed to the conversion process if I confirm all of the tables with errors are, indeed, empty?
The fix for bug 8743409 is not included in the 184.108.40.206 release. Please check the DMU supported configurations. In order to use DMU on any versions of the database before 220.127.116.11, you need to apply a prerequisite server-side patch on one of the supported configurations. If you can upgrade to 18.104.22.168, there is no need to apply additional patches and the aforementioned fix is included.