As I suspected - your function node is still sending a payload (of true or false) to the slider - which it then interprets as "no data". You need to clear out the msg.payload, or use a change node instead to "move" the payload to enabled.
Here is your flow edited.
[{"id":"70dae570.6fe52c","type":"debug","z":"a3c2a16b.fa113","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","targetType":"msg","x":570,"y":1480,"wires":[]},{"id":"bc5f9e77.e136b","type":"ui_slider","z":"a3c2a16b.fa113","name":"setpoint UI","label":"Setpoint °C","tooltip":"","group":"3061152d.046afa","order":3,"width":"7","height":"1","passthru":true,"outs":"end","topic":"","min":"18","max":"26","step":"0.1","x":550,"y":1600,"wires":[[]]},{"id":"be5d18e5.f1ae08","type":"function","z":"a3c2a16b.fa113","name":"enable/disable","func":"msg.enabled = msg.payload;\nreturn msg;","outputs":1,"noerr":0,"initialize":"","finalize":"","x":340,"y":1580,"wires":[[]]},{"id":"902e0524.c8d728","type":"ui_slider","z":"a3c2a16b.fa113","name":"Value Provider","label":"Setpoint °C","tooltip":"","group":"3061152d.046afa","order":3,"width":"6","height":"1","passthru":true,"outs":"all","topic":"","min":"18","max":"26","step":"0.1","x":340,"y":1540,"wires":[["bc5f9e77.e136b","70dae570.6fe52c","1a837a6a.c65de6"]]},{"id":"1a837a6a.c65de6","type":"ui_text","z":"a3c2a16b.fa113","group":"3061152d.046afa","order":5,"width":"2","height":"1","name":"Text Value","label":"","format":"{{msg.payload}}","layout":"row-spread","x":550,"y":1540,"wires":[]},{"id":"84e4201d.0d4fe","type":"ui_button","z":"a3c2a16b.fa113","name":"","group":"3061152d.046afa","order":6,"width":0,"height":0,"passthru":false,"label":"Enable","tooltip":"","color":"","bgcolor":"","icon":"","payload":"true","payloadType":"bool","topic":"","x":160,"y":1560,"wires":[["15893070.d3514"]]},{"id":"8e2be221.0bc2a","type":"ui_button","z":"a3c2a16b.fa113","name":"","group":"3061152d.046afa","order":7,"width":0,"height":0,"passthru":false,"label":"Disable","tooltip":"","color":"","bgcolor":"","icon":"","payload":"false","payloadType":"bool","topic":"","x":160,"y":1620,"wires":[["15893070.d3514"]]},{"id":"15893070.d3514","type":"change","z":"a3c2a16b.fa113","name":"","rules":[{"t":"move","p":"payload","pt":"msg","to":"enabled","tot":"msg"}],"action":"","property":"","from":"","to":"","reg":false,"x":350,"y":1620,"wires":[["bc5f9e77.e136b","70dae570.6fe52c"]]},{"id":"3061152d.046afa","type":"ui_group","z":"","name":"Studio","tab":"f74bf3e6.eaf5e","order":2,"disp":true,"width":"7","collapse":true},{"id":"f74bf3e6.eaf5e","type":"ui_tab","z":"","name":"Stanze","icon":"dashboard","order":2,"disabled":false,"hidden":false}]