Conversion failed when converting date and/or time from character string

Have the code below but i am getting an error message. The error message is "Msg 241, Level 16, State 1, Line 2 conversion failed when converting date and/or time from character string.

Can I get help from someone?

select

UAMicroDate=max(Case
When (convert(datetime,cast(lab.CollDate as varchar(255)),101)cast('1901-01-01' as varchar(255)) and
convert(datetime,cast(lab.CollDate as varchar(255)),101) is not null) Then 'Collection Date'
When (convert(datetime,cast(lab.ResultDate as varchar(255)),101) cast('1901-01-01' as varchar(255)) and
convert(datetime,cast(lab.ResultDate as varchar(255)),101) is not null)
Then 'Result Date' Else convert(datetime,cast(isnull(enc.date, 0)as varchar(255)),101) End),
enc.date, datecheck=ISDATE(enc.date),

enc.patientid pid

from labdata lab

inner join labdatadetail result on result.reportid=lab.reportid
inner join...