Since last week I am no longer able to edit questions on Stack Overflow. Is there any particular reason for this? How can I fix that issue?
It may be that you have too many suggested edits that have been rejected? On your suggestion page it shows 32 rejected out of 81 suggested. You might simply have to wait a few days before making any new suggested edits.
I suspect that the reason you've had so many edits rejected is that you've been only adding trivial tags to questions, one at a time, without making any improvements to the question body or title, with the same exact edit summary (misspelled, ironically).
Many people consider this a waste of time, not an improvement, and therefore reject the edit.
For instance, this edit suggestion that was approved: http://stackoverflow.com/revisions/7010083/2
All you did was add the whitespace tag, which isn't a great retag, but it's valid. You could have improved the question much further, correcting the grammar and punctuation, and making the title more meaningful. Right now it appears that more than half of your reputation has been earned in edits, so you are creating a lot of work for other users (who must review and approve/reject the edit) with very little improvement to the site.