I have the exact same problem on Ubuntu 3.18.04 with hplip 3.19.12 installed. It worked fine for a few months but things went wrong a few days ago (scanner now only works in gray mode and up to 200 dpi). So maybe an update caused a regression ?
I updated to hplip 3.20.3 and 3.20.3 plugin but to no avail. I also tried the solution described in #61 but it didn't help (simple scan simply freezes completely).
I have the exact same problem on Ubuntu 3.18.04 with hplip 3.19.12 installed. It worked fine for a few months but things went wrong a few days ago (scanner now only works in gray mode and up to 200 dpi). So maybe an update caused a regression ?
I updated to hplip 3.20.3 and 3.20.3 plugin but to no avail. I also tried the solution described in #61 but it didn't help (simple scan simply freezes completely).