E_ADEPT_DOCUMENT_TYPE_UNKNOWN (former answers in forum didn't help)

unable to open an .acsm file (related to an ePub file to be downloaded)

though title of eBook is displayed in media shelf

error code "E_ADEPT_DOCUMENT_TYPE_UNKNOWN" appears

Adobe Digital Edition closes

check: my Adobe-ID ist ok, verified

test: other ePub-files (e.g. free ones from Adobe) are displayed

I asked publisher of purchased ePub to create a new .acsm file (in case of damaged file)

BUT: second version of book appears as a titel

but still ePub file can't be downloaded or displayed

(loading process window in Background being coverd by error code window "E_ADEPT..."

Adobe Digital closes down.

What to do?