Employment and occupational disability pensions are partially taxable, just like other pensions from the statutory pension insurance.
Additional information on this can also be found in the question "Does a pensioner have to pay tax on his retirement pension from the statutory pension insurance?".
If the employment and occupational disability pension is converted into a standard old-age pension, the originally determined tax-free pension portion must still be taken into account.
Note
You can also find information about the Retirement Income Act in the "Tax Tips for Seniors".