I've only had my DP24 a few weeks so can't speak to any known/obvious solutions but I have fixed several non-working buttons on my Akai MPC's over the years.
If Tascam used tact switches under the buttons (like Akai did with the MPC) it may be as simple as replacing a $1 part.
Of course, your failure may be harder to troubleshoot....but as an electronics technician, I would carefully disassemble the unit and visually inspect around that button/switch.
Edit: Some older 2488 threads show that people have temporarily fixed unresponsive buttons with contact cleaner/deoxit, but again, not something I've experienced (just throwing you some ideas)
Good luck and please let us know if you figure it out!