Not unusual at this time of year with road salt, damp, and general dirt. I tend to leave the car in gear when it's frosty (on the flat) to avoid stressing the handbrake cable and dreaded sticky piston syndrome!
I did this job back in the Autumn and its been fine since. I replaced the rear calliper seals too as its not unusual to get a sticky piston as well as the sloppy handbrake. The kit is inexpensive from main dealer and allows you to do both rear brakes. It includes new rubber dust sleeves, 'o' ring brake seal, etc. Soak the components in the solution the comes with the kit before assembly.
Remove road wheels, brake carrier, etc. Pump brake to extend the piston from the bore. I used a brake pipe clamp so fluid loss was minimal. Remove piston bore, clean up, etc. You'll need the wind back tool to reinstall. Insert the new 'o' ring seal and lightly lube with a little brake fluid. Insert the new dust sleeve on the piston bore and lightly lube with Lockheed red grease. Bit fiddly to install, but patience is key. Remove the brake clamp. Open the brake bleed valve and wind the caliper in. Close off nipple, etc. You'll need to bleed brakes afterwards to expel any air.
Clean the brake carrier and lube the slider bolts. Don't use copper-ease. Again, red grease is fine.
Now the handbrake itself. Release the handbrake. Think the bolt that holds it on is 10-12mm from memory. Pop it off and remove the spring. There's a little Allen key bolt that holds the mounting plate to the back of the calliper. Ensure its clean and the hex tool sits in it properly before trying to release it or you'll round it off.
Pop it off too. Thoroughly clean the back of the calliper, taking care not to damage the rubber seal. Use a decent marine grease, Duckhams yellow grease is perfect and lube the area before putting the mounting plate back on. Now put the spring back on (having wire brushed it and lightly greased it), then the final 10-12mm nut and tighten. Don't over-torque it.
While you're there check the handbrake cable itself. Not unusual for it to corrode at the end. You can lube this area slightly too. Any doubt about the condition - then replace. Similarly, if this does't work for the handbrake and its sloppy, then in needs to be disassembled inside the caliper which means removing the circlips, etc, which is more hassle than its worth. Go for an exchange unit from Audi.
Hope that helps.
More details here from an early A6 regards rear brake caliper dismantle (handbrake). I'm sure it's the same for a B5.
rear caliper rebuild guide - AudiWorld Forums