The error is caused by this change 7c58aca7b4031b153df49c98bda56b881b0fe950.
When the commit is reversed it works properly.
The Flexform is used in TCA configuration like this:
$TCA['tx_cal_event']['types'] = array(
// Default event
'0' => array('showitem' =>
'--div--;LLL:EXT:cal/locallang_db.xml:tx_cal_event.general_sheet,type, title;;1;;,--palette--;LLL:EXT:cal/locallang_db.php:tx_cal_event.start;5,--palette--;LLL:EXT:cal/locallang_db.php:tx_cal_event.end;6,calendar_id,category_id,' . ($useTeaser ? 'teaser;;;richtext:rte_transform[flag=rte_enabled|mode=ts_css],' : '') . 'description;;5;richtext:rte_transform[flag=rte_enabled|mode=ts_css],
--div--;LLL:EXT:cal/locallang_db.xml:tx_cal_event.recurrence_sheet, freq;;2;;, byday, bymonthday, bymonth, rdate_type;;7;;, deviation, exception_cnt,
--div--;Veranstaltungsort,myextension_cal_location_contact_uid,location,location_link,
--div--;Veranstalter,myextension_cal_organizer_contact_uid,organizer,organizer_link,
--div--;Leitung,myextension_cal_director_contact_uid,myextension_cal_director_contact_alt,
--div--;Eigentümer,myextension_cal_owner_contact_uid,
--div--;LLL:EXT:cal/locallang_db.xml:tx_cal_event.files_sheet,myextension_cal_fal_images,myextension_cal_fal_attachments,
--div--;LLL:EXT:cal/locallang_db.xml:tx_cal_event.other_sheet, monitor_cnt, shared_user_cnt'
),
// Concert
'5' => array('showitem' =>
'--div--;LLL:EXT:cal/locallang_db.xml:tx_cal_event.general_sheet,type, title;;1;;,--palette--;LLL:EXT:cal/locallang_db.php:tx_cal_event.start;5,--palette--;LLL:EXT:cal/locallang_db.php:tx_cal_event.end;6,calendar_id,category_id,' . ($useTeaser ? 'teaser;;;richtext:rte_transform[flag=rte_enabled|mode=ts_css],' : '') . 'description;;5;richtext:rte_transform[flag=rte_enabled|mode=ts_css],
--div--;LLL:EXT:cal/locallang_db.xml:tx_cal_event.recurrence_sheet, freq;;2;;, byday, bymonthday, bymonth, rdate_type;;7;;, deviation, exception_cnt,
--div--;Konzertdetails,myextension_cal_additional_fields,
--div--;Veranstaltungsort,myextension_cal_location_contact_uid,location,location_link,
--div--;Veranstalter,myextension_cal_organizer_contact_uid,organizer,organizer_link,
--div--;Leitung,myextension_cal_director_contact_uid,myextension_cal_director_contact_alt,
--div--;Eigentümer,myextension_cal_owner_contact_uid,
--div--;LLL:EXT:cal/locallang_db.xml:tx_cal_event.files_sheet,myextension_cal_fal_images,myextension_cal_fal_attachments,
--div--;LLL:EXT:cal/locallang_db.xml:tx_cal_event.other_sheet, monitor_cnt, shared_user_cnt'
),
// Church service (Gottesdienst)
'6' => array('showitem' =>
'--div--;LLL:EXT:cal/locallang_db.xml:tx_cal_event.general_sheet,type, title;;1;;,--palette--;LLL:EXT:cal/locallang_db.php:tx_cal_event.start;5,--palette--;LLL:EXT:cal/locallang_db.php:tx_cal_event.end;6,calendar_id,category_id,' . ($useTeaser ? 'teaser;;;richtext:rte_transform[flag=rte_enabled|mode=ts_css],' : '') . 'description;;5;richtext:rte_transform[flag=rte_enabled|mode=ts_css],
--div--;LLL:EXT:cal/locallang_db.xml:tx_cal_event.recurrence_sheet, freq;;2;;, byday, bymonthday, bymonth, rdate_type;;7;;, deviation, exception_cnt,
--div--;Gottesdienstdetails,myextension_cal_additional_fields,
--div--;Veranstaltungsort,myextension_cal_location_contact_uid,location,location_link,
--div--;Veranstalter,myextension_cal_organizer_contact_uid,organizer,organizer_link,
--div--;Leitung,myextension_cal_director_contact_uid,myextension_cal_director_contact_alt,
--div--;Eigentümer,myextension_cal_owner_contact_uid,
--div--;LLL:EXT:cal/locallang_db.xml:tx_cal_event.files_sheet,myextension_cal_fal_images,myextension_cal_fal_attachments,
--div--;LLL:EXT:cal/locallang_db.xml:tx_cal_event.other_sheet, monitor_cnt, shared_user_cnt'
)
);
$additionalCalColumns = array(
...
'myField' => array(
'config' => array(
'type' => 'flex',
'ds_pointerField' => 'type',
'ds' => array(
'5' => 'FILE:EXT:my_extension/Configuration/FlexForms/FlexformOne.xml',
'6' => 'FILE:EXT:my_extension/Configuration/FlexForms/FlexformTwo.xml',
),
)
),
...
)