Cannot take the length of Shape with unknown rank
Could you post a longer stack-trace? I think your problem might be related to this recent tensorflow issue:
https://github.com/tensorflow/tensorflow/issues/24520
There's also a simple PR that fixes it (not yet merged). Maybe try it out yourself?
EDIT
Here is the PR:
open tensorflow/python/keras/engine/training_utils.py
replace the following (line 232 at the moment):
if (x.shape is not None
and len(x.shape) == 1
with this:
if tensor_util.is_tensor(x):
x_shape_ndims = x.shape.ndims if x.shape is not None else None
else:
x_shape_ndims = len(x.shape)
if (x_shape_ndims == 1