If a data block in S7-300 have 8 bytes and XBTGT want to write a bit of the final byte, then error will appeared.
The same principle, for S7-224, Memory address max to 31 byte, When XBTGT want to write M31.0, Then error will appeared.

Here are a couple of workarounds we can offer the customer:
1) If M31.7 is needed on the panel, place an integer variable with device address MW30 on the same panel. This will allow M31.7 to be read properly.
Similarily, you can do the same for a Data Block on the S7-300 series.

2) Use M30.7 as the maximum address. In other words, the maximum address is 1 less than specified by the manual.