I also experienced the "Unexpected channel count" error using 1.5.0 build 7492.
This happened on a project of 216 TIFF images. 205 were masked in Photoshop so they included an alpha channel. The remaining 11 images were not masked and included no alpha channel. Photos aligned as usual and a good sparse cloud was created. The "Unexpected channel count" error occurred at the end of Dense Cloud creation.
Although no dense cloud was created, I noticed a depth map had been. I don't use depth maps but I also noticed the Mesh panel said a mesh could be created from the depth map. I tried that, and it worked even though the error occurred for the dense cloud. The Shaded, Solid, and Wireframe models looked fine.
Next, I proceeded to create a textured model. Again, the "Unexpected channel count" error occurred. This time, the Console indicated 11 images contained only 3 channels rather than 4. Those images were then disabled, and a good textured model was created. Still, no dense cloud was ever created.
I plan to create masks for those 11 images that originally didn't have them, and rerun the project with all images having masks.
Before the update to 1.5.0, I never had a problem mixing masked and unmasked TIFF images.
From the first texture attempt:
2019-01-26 06:24:55 - 100% done by [CPU 1] with timings: 2267 s = 0% fetch + 1% init + 77% hist (96% GPU) + 0% init + 21% iters (81% + 19%) + 0% updating
2019-01-26 06:24:55 IO utilization: 1%
2019-01-26 06:24:55 Level done in 2268.3 sec
2019-01-26 06:24:55 Surface estimated in 2454.16 sec
2019-01-26 06:24:55 Extracting surface...
2019-01-26 06:26:06 8513494 vertices total
2019-01-26 06:26:06 17026857 faces total
2019-01-26 06:26:06 done in 71.457 sec
2019-01-26 06:26:06 Calculating colors...
2019-01-26 06:26:09 Warning! Image ignored because it has 3 channels, but 4 channels expected!
2019-01-26 06:26:09 Warning! Image ignored because it has 3 channels, but 4 channels expected!
2019-01-26 06:26:09 Warning! Image ignored because it has 3 channels, but 4 channels expected!
2019-01-26 06:26:10 Warning! Image ignored because it has 3 channels, but 4 channels expected!
2019-01-26 06:26:10 Warning! Image ignored because it has 3 channels, but 4 channels expected!
2019-01-26 06:26:12 Warning! Image ignored because it has 3 channels, but 4 channels expected!
2019-01-26 06:26:12 Warning! Image ignored because it has 3 channels, but 4 channels expected!
2019-01-26 06:26:13 Warning! Image ignored because it has 3 channels, but 4 channels expected!
2019-01-26 06:26:13 Warning! Image ignored because it has 3 channels, but 4 channels expected!
2019-01-26 06:26:13 Warning! Image ignored because it has 3 channels, but 4 channels expected!
2019-01-26 06:26:15 Warning! Image ignored because it has 3 channels, but 4 channels expected!
2019-01-26 06:28:20 done in 133.252 sec
2019-01-26 06:28:20 Decimating surface...
2019-01-26 06:28:35 decimated to 479561 faces
2019-01-26 06:28:35 done in 15.277 sec
2019-01-26 06:28:36 Finished processing in 29467.4 sec (exit code 1)