It sounds like whatever you are pasting in the "sweep" dialog is not a valid private key(s). When you exported your private key... did you use a password or did you leave the password blank?
Does your password file look like:
# KEEP YOUR PRIVATE KEYS SAFE !
# Anyone who can read this file can spend your bitcoin.
#
# Format:
# <Base58 encoded private key>[<whitespace>[<key createdAt>]]
#
# The Base58 encoded private keys are the same format as
# produced by the Satoshi client/ sipa dumpprivkey utility.
#
# Key createdAt is in UTC format as specified by ISO 8601
# e.g: 2011-12-31T16:42:00Z . The century, 'T' and 'Z' are mandatory
#
Labc123abc123abc123abc123abc123abc123abc123abc123abc 2013-01-01T12:00:00Z
Kdef456def456def456def456def456def456def456def456def 2013-01-01T12:00:00Z
# End of private keys
or does it look like:
U2FsdGVkX1+YqTr9vabcfVlfKlCVTjR7kEg/xD4rEUB1tBBQm6chWSOSIUyJKK/9yjxFSHkge/xI
UCFq3MfFKqUbRnYcmOhNLBabcGRrbvsx3AawNvzp1sdHFXNvH3I+INMtYaoTjMEJVkQM8KnaUwZI
GE5PYKtMHncv+2pYA1/3Lg09hPA++jbvwL7L+NH6ZcljvYZxseu+3M45YNK1CJ/4xQJ+Yvo6OVqJ
nL7xIR0=
it looks like the first, i have only one private key, so from your example i paste to the sweep window, my key line that similar to this part:
Labc123abc123abc123abc123abc123abc123abc123abc123abc
is that correct?