Easy one, this.
I have a Bosch dryer of similar vintage, probably the same model, that deve loped this fault a while back. It's fluff and crap in the trough that colle cts the condensate and/ or in the pick-up end of the pipe that takes the wa ter to the collection container at the top.
Open the condenser unit flap and remove the condenser. On the floor of the now empty condenser bay, there are one or two (I forget) removable plastic covers. It's a bit of a fiddle to get them off, but if you persist, it can be done without breaking. I use a couple of stubby blade screwdrivers to ho ld the (quite stiff/brittle) retaining lugs in the open position while wigg ling the removable bits. You need something short because the height of the bay is only a few inches.
When the cover or covers are off, you ought to be able to see the collectio n trough, which will be full of a thick, fragrant broth of water and fluff. Use a torch if necessary. Grope around and pull out what you can (a washin g up bowl is useful to deposit the crap). When what remains is mostly liqui d, use a cloth to soak it up, with repeated wringing out.
The trough should now be clear and depending upon hand size and dexterity, you might be able to grope around and feel the end of the collection pipe t o check that it's clear. I think you might be able see it, too, but I can't recall.
At this point, the machine will probably work again. If the collection pipe is blocked, you can reverse flush or poke some high strain (i.e. thick) fi shing line down from the top. This is done by removing the machine lid (Tor x screws at the top rear, IIRC), whereupon the collection container and the top of the pipe that fills it becomes visible. You can detach the tubing t o flush or poke things down it.
If you are OCD, take the machine outside, tilt it forward with the collecti on trough cover still open, then hose the condenser bay out to get it all s queaky clean. Give a really good final tilt forward (50+ degrees) to drain the last of the water you squirted in.
Another good bit of maintenance to do every 5 years or so is to remove the back of the machine (loads more Torx, so use a drill/driver, otherwise you will get blisters), including the cover for the heating element, and remove the debris. Manually first, then vacuum cleaner with brush attachment. Tak e care not to bugger the heating element, which looks like a bit of card wi th wire wrapped round it. Doing this clean maintains good air flow through the heating element and go od heat transfer from it. It also prevents fluff build up acting as insulat ion on the temperature sensor (a little round device, about an inch diamete r, with a couple of wires leading from it).
All the best.
Terry.