TYPO3 Core

Yes and No.
Also div2007 has it's own function for dealing with getLL and since the structure of pibase->LOCAL_LANG has changed, all these lib functions are broken.

sr_feuser_register for example only displays "Array" instead of the labels. (Already contacted Franz concerning this bug in div2007)

If I change the behavior of sL() within tslib_fe by removing the final return and adding this logic then, it fixes the bug:

if (!$this->LL_labels_cache[$this->lang][$input]) {
    $output = $input;
} else {
    $output = $this->LL_labels_cache[$this->lang][$input];
}
return $output;

So, is this a bug in the way the seminars extension, a bug in the ameos_formidable extension or a Core bug?

Ok seems to be a different story here then.
(Didn't have a look at ameos_formidable, but in the div2007 case it's not the Core's fault)

The sL() method in 4.5:

        /**
         * Split Label function for front-end applications.
         *
         * @param       string          Key string. Accepts the "LLL:" prefix.
         * @return      string          Label value, if any.
         */
        function sL($input)     {
                if (strcmp(substr($input,0,4),'LLL:'))  {
                        $t = explode('|',$input);
                        return $t[$this->langSplitIndex] ? $t[$this->langSplitIndex] : $t[0];
                } else {
                        if (!isset($this->LL_labels_cache[$this->lang][$input])) {      // If cached label
                                $restStr = trim(substr($input,4));
                                $extPrfx='';
                                if (!strcmp(substr($restStr,0,4),'EXT:'))       {
                                        $restStr = trim(substr($restStr,4));
                                        $extPrfx='EXT:';
                                }
                                $parts = explode(':',$restStr);
                                $parts[0] = $extPrfx.$parts[0];
                                if (!isset($this->LL_files_cache[$parts[0]]))   {       // Getting data if not cached
                                        $this->LL_files_cache[$parts[0]] = $this->readLLfile($parts[0]);
                                }
                                $this->LL_labels_cache[$this->lang][$input] = $this->getLLL($parts[1],$this->LL_files_cache[$parts[0]]);
                        }
                        return $this->LL_labels_cache[$this->lang][$input];
                }
        }

and in 4.6:

        /**
         * Split Label function for front-end applications.
         *
         * @param       string          Key string. Accepts the "LLL:" prefix.
         * @return      string          Label value, if any.
         */
        function sL($input)     {
                        // If cached label
                if (!isset($this->LL_labels_cache[$this->lang][$input])) {
                        $restStr = trim(substr($input,4));
                        $extPrfx='';
                        if (!strcmp(substr($restStr,0,4),'EXT:'))       {
                                $restStr = trim(substr($restStr,4));
                                $extPrfx='EXT:';
                        }
                        $parts = explode(':',$restStr);
                        $parts[0] = $extPrfx.$parts[0];
                                // Getting data if not cached
                        if (!isset($this->LL_files_cache[$parts[0]])) {
                                $this->LL_files_cache[$parts[0]] = $this->readLLfile($parts[0]);
                        }
                        $this->LL_labels_cache[$this->lang][$input] = $this->getLLL($parts[1],$this->LL_files_cache[$parts[0]]);
                }
                return $this->LL_labels_cache[$this->lang][$input];
        }

I'd say that this is an edge-case because in 4.5 the splitLabel feature allowed a non-existing key to be used (in fact a split label which in this case was not "split" at all) and it worked. In 4.6, split label support has been removed and as such the support for non-existing keys as well.

Xavier Perseguers wrote:

In 4.6, split label support has been removed and as such the support for non-existing keys as well.

So what happens if you call the method with a non existing label? Expected result would be that the label is returned, wouldn't it?

Patch set 2 of change I2847e51fd479d9636ac815c820b2e63cf449d6a7 has been pushed to the review server.
It is available at http://review.typo3.org/6364

Patch set 1 of change I98362a2701384f13fad80832eafc56ba494e8c34 has been pushed to the review server.
It is available at http://review.typo3.org/6368