Most of the times happens on alts. Probabbly some issue with saving settings (potion, mount or path ability) ?

yeah got the same problem on my alt. need to deactivate action bar to change / regroup skills

u can easily fix it. reset the action bar addon via the "reset settings" once.
when u open the forgeui options, klick on "addon" to open the forgeui addons tab. there u can reset this one addon.

Issue:
Copying settings between characters results in eventually being unable to change LAS settings.

Details:
This issue is caused by the following items being rendered as 'invalid' values:

Potion(s)
Path Skill(s)
Mount Skill(s)

Easy Fix: All of these need to NOT save which one you last had selected (requiring you to reselect them on every login).

Harder Fix: All of these need to auto-change to the 'empty' icon/value (when you have none) whenever the value is 'invalid'. I looked in the code and it just does a lazy 'goto end' for invalid values. Needs to be changed to specifically set the empty value instead.

I don't have permission from the author/maintainer of ForgeUI to release a tweaked version but I can post here and on Curse and hope people are willing to dig in and edit it themselves until this is officially fixed.

I'm completely unfamiliar with LUA so I'll be testing on my end before posting anything new.
I'll backup and move my settings folder and make a completely fresh account.
Then I will use completely default ForgeUI settings on a character with no mount, recall, gadget, potion, or path skills.
Then I will exit out of the game and check the saved settings to see what values are used for these 'empty' settings.
Then I'll attempt to have ForgeUI intelligently detect these invalid values and set the 'empty' setting appropriately.

If this works, the bug will be hopefully fixed and I can pass along the code here and link it on the Curse comments if anyone comes across it again.

EDIT: Recall, Gadget, and Stance skills are not saved and have no effect on this bug apparently.

WORKAROUND to PREVENT this LAS bug from happening.

Before you copy your main's settings over to alts, be sure to do the following:
-Login to the alt so they get folders & settings INI files created for them
-Make the below edits to your ForgeUI_0_Char.xml in your AddonsSaveData directory

Edits for ForgeUI_0_Char.xml ;; open the file in WordPad or your fav text editor (not NotePad)

-Old: "nSelectedPotion" T="n" V="fivedigitnumber"
-New: "nSelectedPotion" T="n" V="0"

-Old: "nSelectedPath" T="n" V="fivedigitnumber"
-New: "nSelectedPath" T="n" V="0"

-Old: "nSelectedMount" T="n" V="fivedigitnumber"
-New: "nSelectedMount" T="n" V="0"

Just find those values and replace the existing five digit number with a 0 (zero) as shown above.
Save the file and NOW you can feel free to copy all your ForgeUI settings as you like ^_^

RESET SETTINGS if your LAS is bugged and you just want to fix it.
Open ForgeUI Options, Click Addons button, then click the ... next to ActionBars and click Reset Settings.

Picture here:
https://www.anony.ws/i/2015/12/10/ForgeUIActionBarReset.png

The downside to resetting is you'll have to go in and set all your options for Action Bars within ForgeUI again. Hide hotkeys, etc etc and set up your Side Bars again.

Mount limit on LAS - Workaround

Backup any files before you change them

1. Open ForgeUI_Actionbars.lua with WordPad or your favorite text editor
2. Search for this line: if nCount < 10 then nCount = nCount + 1 end
3. Replace the number 10 with any number you want higher than 10
4. Save & close file, restart the game and test it out!

Update @ Dec 30 2015:
I've received confirmation that this workaround DOES work and has no major issues with performing it.
Just realize that the more mounts that have to be displayed/shown, the slower that ForgeUI will run and the more memory/CPU it will consume.

I can understand the hardcoded 10 mount limit although I still wish the author had time to put in an option within the settings to change this.

Fixed in v0.5