To reproduce:
- Try to import an rtf document that contains table with RtfFormatProvider Import method.
- Please note that this is not this is not occurring with all tables. The issue will occur when the table is created with the following string:

\trowd\trbrdrt\brdrs\brdrw20\brdrcf1 \trbrdrl\brdrs\brdrw20\brdrcf1 \trbrdrb\brdrs\brdrw20\brdrcf1 \trbrdrr\brdrs\brdrw20\brdrcf1 \clbrdrt\brdrw15\brdrs\clbrdrl\brdrw15\brdrs\clbrdrb\brdrw15\brdrs\clbrdrr\brdrw15\brdrs \cellx3130\clbrdrt\brdrw15\brdrs\clbrdrl\brdrw15\brdrs\clbrdrb\brdrw15\brdrs\clbrdrr\brdrw15\brdrs \cellx6260\clbrdrt\brdrw15\brdrs\clbrdrl\brdrw15\brdrs\clbrdrb\brdrw15\brdrs\clbrdrr\brdrw15\brdrs \cellx9390\pard\intbl\sl276\slmult1\f1 Mere test\f2\cell\cell\cell\row
\intbl\cell\f1 Blabla\f2\cell\cell\row

Workaround:
- Edit the document in Word or WordPad, and then save it.
- Then The document will be correctly imported.

Resolution: 
This issue is addressed in the new version of the control - RadRichTextEditor. Please use the new control instead the RadRichTextBox.