The letter states that one of the CLAMP members has been recently diagnosed with lumbar compression fracture due to sitting on chair for many hours straight with no resting time.
The doctor advised 6 months of rest, otherwise dealing with the pain may become a much more difficult task later.
The members of CLAMP are aware that following that advise will prove to be a very difficult task giving to the fact of currently having two manga series on-going and other projects in development. They thought deeply on the subject and decided to take the treatment seriously. Therefore, there might be a decrease in the page rate of their current works and some side-projects might be put on hold, all so that they can focus on treatment.
They apologize their publishers, readers and those involved in their side-projects and ask for everyone's comprehension.
They finish the letter with a warning for everybody to look after their health during work tasks.
(I think the text also mentions that another member has been recently diagnosed with a hip fracture, which prevents her from spending too much time in front of the computer, but I am not absolutely sure of this one).
I sincerely hope that they get a safe and fast recovery. I think we can expect some breaks in the current series and a decrease in number of pages per chapter. The first half of the next year may turn out to be quieter than usual.
Let's hope for the best.