[TYPO3-seminars] Registration crashes

[Johanna]

Hi,
when I submit the onetimeregistration form it crashes and I get a 
message as following:

Warning: 
require_once(/www/1111_2222/rp-hosting/1234/56789/typo3cms/XXX/t3lib/class.t3lib_htmlmail.php) 
[function.require-once]: failed to open stream: No such file or 
directory in 
/kunden/1111_2222/rp-hosting/1234/56789/typo3cms/kinderzukunftsdiplom/typo3conf/ext/ameos_formidable/api/base/act_redct/api/class.tx_actredct.php 
on line 9

Fatal error: require_once() [function.require]: Failed opening required 
'/www/1111_2222/rp-hosting/1234/56789/typo3cms/XXX/t3lib/class.t3lib_htmlmail.php' 
(include_path='/www/1111_2222/rp-hosting/14075/64074/typo3cms/XXX/typo3/contrib/pear/:.:/usr/local/lib/php') 
in 
/kunden/1111_2222/rp-hosting/1234/56789/typo3cms/XXX/typo3conf/ext/ameos_formidable/api/base/act_redct/api/class.tx_actredct.php 
on line 9

Is there anyone with the same Problem or a idea how to fix it?

TYPO3 4.7.1
seminars 0.9.3
ameos_formidable 1.1.549
onetimeaccount 0.8.0
oelib 0.7.7