2x FrameXML\InterfaceOptionsFrame.lua:625: bad argument #1 to 'strlower' (string expected, got boolean)
[string "=[C]"]: ?
[string "@FrameXML\InterfaceOptionsFrame.lua"]:625: in function `InterfaceOptions_AddCategory'
[string "@BuffTimers\Options-Options.lua"]:47: in main chunk

Locals:
(*temporary) = true
(*temporary) = "string expected, got boolean"

@Mathizsias Thank you for your help. I just wanted to report the bug here, as it seems to be a good place for it. ;)
Hope this helps improving the addon, because I like it. It got posted on curseforge, as well as @boktai1000 mentioned.

This issue was fixed in v1.5.7. Are you still experiencing this bug? @Blumberg1337

@sandervspl No, just forgot to close this issue. Thanks!

Posted it as well here: https://www.curseforge.com/wow/addons/bufftimers?comment=31

Message: Interface\FrameXML\InterfaceOptionsFrame.lua:625: bad argument #1 to 'strlower' (string expected, got boolean)
Time: Mon May 23 14:49:05 2022
Count: 1
Stack: Interface\FrameXML\InterfaceOptionsFrame.lua:625: bad argument #1 to 'strlower' (string expected, got boolean)
[string "=[C]"]: ?
[string "@Interface\FrameXML\InterfaceOptionsFrame.lua"]:625: in function `InterfaceOptions_AddCategory'
[string "@Interface\AddOns\BuffTimers\Options.lua"]:47: in main chunk

Locals: (*temporary) = true
(*temporary) = "string expected, got boolean"

Message: (null)
Time: Mon May 23 14:49:11 2022
Count: 1

Message: (null)
Time: Mon May 23 14:49:11 2022
Count: 1

Message: (null)
Time: Mon May 23 14:49:11 2022
Count: 1

Reverting back to a previous release solved this particular issue for me.

https://github.com/sandervspl/BuffTimers/releases/tag/v1.5.5

You can then set it to be ignored in WoWUP or the CurseForge client.