hluebeck,
I’ve checked on a Mac with OS X High Sierra and find that the Option-5 key combination produces the infinity symbol (“∞”). There seems to be no way to change this, so I’m confused about your last reply. Do you have a special keyboard, where the Option key mappings are different?
You can look at the various keyboard mappings by checking the “Show Input menu on menu bar” option in the Input Sources tab of the Keyboard preferences (in System Preferences), then click the Input Menu icon on the right hand side of the Mac’s menu bar. In the keyboard that’s shown, you can click an Option key and see what characters each key produces with the Option key held down. What do you see for the “5” key, when the Option key is held down?
The only other possibility I can think of is that Language and Region settings are affecting this in some way. Could you let us know what settings you have there and what version of OS X you are using? I’m glad you’ve found a way round this but we’d like to understand what was causing the original problem, as we might be able to address it in the product.
Thanks for any assistance you can provide in helping us track this down.