I found another thread with a similar issue that also has a couple of possible ways to fix it:

https://community.intuit.com/questions/790228-there-are-suddenly-black-lines-and-boxes-in-lines-and-…

Resolution for 'black lines issue: 'I have black lines all over the place. Do I have a corrupted file?' available: Yes.

The black lines issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the black lines issue yourself by looking for a resolution described in the article. If it is a complex black lines issue or you are unable to solve the issue, you may contact us by clicking here or by using other black lines support options.

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