on experience of the following error message

fatal error: cannot use string offset as an array in /opt/typo3_src/typo3_src-4.2.1/typo3/class.db_list.inc on line 696

(e.g., happening when you switch to list view) try the following: click on the typo3 installation tool icon. if it should be deactivated, re-activate it by installing the (empty) file as mentioned in the error message. (you can use quixplorer for that purpose if installed.)

select "compare with $tca" from the menu point "database analyser" and watch out for the error messages. in the case at hand, two error messages pertaining to tt_content were seen. start phpmyadmin (from typo3 backend if installed there, from the browser otherwise) or access the database using sql commands otherwise. change the respective tables (here tt_content) by adding entries as required (tx_dmcimagealttext and tx_dmcimagetitletext respectively). save and check with "compare with $tca" again. in the present case, the database inconsistency was removed and the problem solved.